I often see trees disfigured by bad attempts at pruning. Cutting limbs arbitrarily results in wounds that stay open longer than necessary (if they heal at all) and ugly proliferations of new-growth shoots.
It only takes a little more energy (mostly mental) to prune in a way that helps the tree recover from the injury. Make your cut nearly flush with the trunk. You especially want to preserve the top of the bifurcation - you'll often see this as an area where the bark is rough and flakes off, almost as if the barks of the trunk and branch are "colliding". I no longer remember much of plant physiology but there's something magic about that part that can initiate new growth to cover the wound. Your cut should look something like this:
After a few years (exactly how long it takes depend on the size of the wound), the tree will heal the wound, by growing tissue to cover it from outside in. Here's an example of a properly made cut on the same tree as above, after a few years' healing:
Eventually the wound closes completely, and over time the former wound site becomes almost unnoticeable.
If anyone has any particularly egregious examples of poor pruning, I'd like to add an image of it here.