Pretty easy to reproduce.
1) Attack an enemy with an EM weapon.
2) Each time you hit f thereafter, the target will be right over the enemy, per usual.
3) Corrupt the enemy a bit.
4) Have the enemy do a misfire via corruption, one of the new beta 4 mechanics
5) hit f again. Instead of being right over the enemy, your target square will be where the enemy's misfire attack went.
Ha this is awesome, thanks for reporting it Happylisk!
In fact technically this "bug" has been in the code for ages, but there was no way to trigger it. Any shot aimed at a non-occupied spot was assumed to be fired by Cogmind, in which case the UI could freely remember the target. But obviously Beta 4 added the chance for any bot to misfire, which means technically the AI may actually fire at a non-occupied spot (they never would before!), and the UI picked up on that and thought you did it :P. So this won't happen in the misfire cases where the corrupted AI fires at a robot.
In any case, fixed for Beta 5.