1). When removing the pin with either left or right click and immediately following up with lever release
If you pull the pin and release the lever too fast, it ends up switching to the opposite throw intended. For instance, left clicking while holding a nade (to initiate over-hand throw) and immediately following it with a right click (to release lever) switches it to an underhand throw, thus completely changing the intended throw.
2). Switching weapons/grenades via scroll wheel
When using scroll wheel too fast/often to switch weapons/grenades, it often doesn't end where indicated (Such as, scrolling to a grenade and then your knife and then back to your grenade fast enough and it might just stay at your knife being equipped (despite showing that your grenade is selected).
3). Superman prone throwing
When you throw a grenade while prone (C4, IED, nades, smokes), It ends up going way further than intended (and quite frankly beyond the laws of physics)