Let's say you kill Newb Rats. If you change your mob up to Small Rats, but you get captcha right away, after you solved it it will shown Newb Rat on mob drop box. Now if you are like "I picked Small Rats already let's just hit auto fast" you actually kill Newb Rats :S
I have countered this situation multiple times when my quest is ended and I test new mob, but then this happens it's pretty annoying.
Hmm, I'm not certain that this is necessarily a bug but rather a feature. The game only remembers the last killed mob, not the last selected mob. That's always been the case. For instance, if you select a new test mob and can't kill it once, it'll still recall the last killed mob.