I remember waiting to pick up a Bellsprout in Blue so I can have him higher level, forgetting completely that the game always removes the lowest move on their move list, which meant I got stuck with a Bellsprout who doesn't know the awesome Vine Whip and instead has to nail an opponent with status spores before spamming Wrap, which takes forever since the mon isn't exactly a powerhouse. Suffice to say I dropped him and went back to Cerulean City to get a proper one.

In a similar vein, I've also had the misfortune of leaving some good Pokemon at the Daycare center to get charged up in levels only to forget the Daycare attendant will also wipe out the earliest abilities on their list for the new ones. This is especially annoying with Fire-Types as many of them learn Ember surprisingly early and won't gain the better Flamethrower or okay Flame Spin until endgame. Vulpix is especially vulnerable as she has lackluster offense beyond special attack so losing Ember makes them useless offensively.