Robo Koopa - S1-E52
Plot hole: Near the end, Robo Koopa sets down Dr. Nerdnik after he had just captured and held him hostage. Why would he do that? (00:11:55)
Another Mother - September 30, 1981 - S2-E13
Plot hole: Sam wallops both kidnappers and rescues the son. They then leave the two child molesters unconscious beside their van. No one calls the police or has them arrested, so they're free to kidnap another kid. Sam doesn't leap until after an epilogue in which the loose ends are supposed to be resolved - but this question is left unanswered. It's not excused by a "just because we didn't see it happen doesn't mean it didn't" defense. The viewers shouldn't be left with a dangling plot hole, and one line could have cured this one.
Gakkou wa senjouda! Taiketsu Ryouma vs Ryouga - S1-E8
Plot hole: In his flashback, Ryoga emerges from the sea and faints on the beach of Okinawa. That's quite the big oversight in plot, since Ryoga, as revealed in the following episode, has a curse similar to Ranma's and he would have emerged from the sea in his animal form and not as 'himself'. (00:07:20)