If the hole you cut for an electrical outlet in drywall is a bit off, don’t just hope that the cover plate will hide the mistake. Instead, fill the void with expanding foam. The foam sticks to electrical boxes and raw drywall edges just fine. Once the foam is hardened, it can be trimmed to below flush and mudded over. Without the foam, the mud would simply crack off with light pressure. The foam gives good backing for the mud and also helps to block air leaks.
Jim McAllister, Portland, OR