The community forums and Web pages for Blender are brimming with questions from new users. Many of them are the same question, or derivatives of the same question. Following are some of the most common ones and give you solutions to them so that you don't have to dig through these websites.
Blender's interface is weird or glitchy
Blender uses OpenGL, an accelerated 3D programming library, for its entire interface. Because of the extensive use of OpenGL (and some rather old versions of it), Blender often uses parts of the library that other programs may never touch. Depending what video card you have in your computer, the drivers for that card may not effectively implement these little-used library features that Blender needs.
On some machines, Blender may run very slowly, or you may see weird screen glitches around the mouse pointer or menus. The first thing to check is the drivers for your video card. Go to the website for the manufacturer of your video card to see whether any updates are available.
You may want to turn off any fancy effects that your operating system adds, such as transparent windows, shadows on the mouse cursor, or 3D desktop effects. Within Blender itself, go to the System section in User Preferences (Ctrl+Alt+U) and find the Window Draw Method drop-down menu in the center column. Play with the different options here to see which one works best for you. Blender updates immediately when you make the change, so you don't need to restart anything.
A notorious black stripe appears on models
Often when modeling, you run into a situation where a strange black crease goes along some edges. The stripe is usually most apparent when modeling with the Subdivision Surface modifier turned on and you're looking at your mesh in Solid viewport shading. What's happening here is that the normals for one of the faces adjoining this edge are pointing the wrong direction.
Usually, the quickest way to fix this problem is to have Blender recalculate the normals for the model and attempt to have them all face outside. To do so, go into Edit mode, select all, and press Ctrl+N. Typically, pressing Ctrl+N alleviates all issues.
Objects go missing
Occasionally, you might run into a problem where not everything shows up in your 3D View, even though you're positive you didn't delete anything. The first thing to do is to make sure that nothing is hidden. Pressing H in the 3D View hides whatever you've selected, and it's easy to accidentally hit it when you're actually trying to press G and grab an object. Fortunately, you can unhide all hidden objects pretty quickly by pressing Alt+H.
You can also look in the restrict columns on the right side of the Outliner. If your object is hidden, the first icon — the eye icon — appears closed. Left-clicking the eye icon unhides it. The camera icon on the far right controls whether your object is visible when rendering.
If you're sure that nothing is hidden, next try to make all layers visible and check to be sure that you didn't inadvertently move your object to a different layer. You do so by pressing the Tilde (~) key. You may also want to press Home in the 3D View to bring all objects into view.
One last thing to check is whether you're in Local View, the view that isolates all objects except for a few that you select. The hotkey that toggles this view is Numpad Slash (/), and it can be pretty easy to accidentally hit it when using the numeric keypad to change views.
Edge loop select doesn't work
The issue of Edge Loop Select not working happens the most on Linux machines. The Blender hotkey for doing a loop selection in Edit mode is Alt+right-click. Unfortunately, in Gnome 2 and a few other window managers, this key sequence pops up a menu for controlling the window.
You can fix this issue in two ways. The easiest one is to use Shift+Alt+right-click. You typically use this combination to select multiple loops, but if nothing is selected, it works exactly the same as the Alt+right-click combination.
A better solution is to modify the window manager's settings and bind the function that it ties to the Alt key to another key, like the infamous "super" or Windows key that most modern keyboards have. Because the method varies from one window manager to another, you'll need to consult the documentation on your window manager to see the exact steps on how to do this.
A background image disappears
When using a photographic or drawn reference to base your models on, it's a common practice to load the reference image in the background of the 3D View. However, when working this way, you may orbit your view to do a spot-check and then when you return to side (or front or top or camera and so on) view, the background image may disappear, even though the Background Images panel in the 3D View's Properties region says it's still there.
The answer here is that you're viewing the scene through Perspective view rather than the Orthographic one. Switch back to Orthographic by pressing Numpad 5. The Orthographic view is much more effective at getting a model to match a reference image.
Alternatively, you could an Empty with Image set as its display type. This ensures that your reference image is visible at all times and from all angles.
Zooming has its limits
When working in Perspective view, you may notice that occasionally you can't zoom in on your scene as much as you'd like. This limitation is because you're zooming toward a center point, and you're very near it. You can take advantage of four workarounds:
Place the 3D cursor at the location you'd like to zoom to and press Ctrl+Numpad Dot (.).
Select the object (or sub-object element, like a vertex or edge in Edit mode) that you want to zoom in on and press Numpad Dot (.).
Try popping quickly into Orthographic view by pressing Numpad 5.
Enable the Auto Depth check box in the Interface section of User Preferences (Ctrl+Alt+U).
Lost simulation data
Blender saves some simulation data to your hard drive. Unfortunately, if that simulation data isn't where Blender expects it to be, your simulation won't show up in your .blend file. Generally, lost simulation data happens for one of three reasons:
You work on more than one computer.
You accidentally changed or deleted the path to the simulation data.
You're using the /tmp directory for your simulations.
Objects don't appear when rendering
Sometimes, when you render, you might notice that some objects are perfectly visible in the 3D View, but they don't show up in your render. There are a few reasons that this may happen:
Camera clipping.
The fix for this is to either change the position of the object that's too far away or increase the end clipping distance for your camera object (Camera Properties→Lens→Clipping).
Restricted rendering.
Fortunately, the fix is easy. Go to the Outliner, find your object, and re-enable renderability on it by left-clicking the rightmost icon next to it (the icon looks like a camera).
Non-rendered layers.
To fix this issue, go to Render Layers Properties and double-check the scene layers that you have assigned to your render layers. If you know which layers an object is on, you should be able to ensure those layers are enabled.
No GPU rendering for cycles
One of the biggest appeals for using Cycles as a renderer is its ability to leverage your video card's GPU to speed up calculations (and, by extension, the overall rendering process). Unfortunately, depending on your computer hardware, you may not be able to take advantage of this speed boost.
The first thing to check is whether you're currently using GPU computing at all. Your next step is to look at the System section of User Preferences (Ctrl+Alt+U). At the bottom of the leftmost column is a little section labeled Compute Device. The first set of radio buttons beneath that should give you the options of None, CUDA, or OpenCL.
You want to pick CUDA (currently, that's the only one where Cycles' GPU computing works reliably). CUDA is a GPU computing technology developed by NVIDIA to run on their cards. Once you pick CUDA, you should be able to pick your specific video card from the drop-down menu below those radio buttons.
Funky deformations in animation rigs
This isn't necessarily specific to animation rigs, but animation rigs are where you most often see it. The effect is easy to recognize. You'll have a mesh set up to be controlled by an armature, for example; when you grab or rotate any of the bones, the mesh stretches, skews, and scales wildly, like it isn't even really being controlled by the armature at all.
A few different things may cause this behavior, and it may happen any time you add a deforming modifier (such as Armature, Curve, Lattice, or Mesh Deform) or nearly any constraint to an object. If this is happening to you, check for these red flags:
Non-applied transformations.
Doubled-up modifiers.
Cyclic dependencies.
