When there is a lag in response and you click a button or menu item, it changes to something else in the blink of an eye and you've clicked something completely different.
+1 This is not just for apps, it also happens with websites. I am not a web guy, so I don't know the jargon but web pages keep changing even after they are done. I tap on something but it just adds another element there and I end up tapping on something else since everything shifts
I remember when the "tableless" concept first appeared a long long time ago and one of the main arguments in favor of implementing it was to prevent the page rearranging itself as it loaded. Look at where we are now...