One golden rule that I learned is that interface elements should not move unexpectedly after the interface has been drawn.
Google is particularly bad for buttons which move between you lifting your thumb and pressing the screen, but they're not alone.
I wish this would be addressed at the OS level. If a target popped into existence less than ~0.25 seconds before it was touched, a touch event shouldn't be generated. Humans reaction times aren't fast enough to hit a button that quickly anyway.
One problem with this: People often learn to touch multiple targets quickly in sequence, because one touch event predictably pops up the target for the next event.
