To forget that one is holding down a key on the keyboard. When performing unusual or complex keypress combinations, it is easy for one to actually forget one is pressing a key, and keep it held down with their finger. When this happens, they may believe that their application is glitching by continually performing one action, such as printing a long line of "z". Depending on the situation, it may actually take a few moments for the user to think about the problem and find the solution to it.
I was wondering why my my pages kept scrolling down without my permission, then I realized I was doing a keyfart on the down arrow.