I've also found a bug whereby the game will crash if you try to catch a monster that has already been captured in the same turn, because there is no longer a target. So two player party members, on both party members turn you use a monster ball item, if on the first party members turn the monster is caught, the second party members turn tries to use the monster ball but they can't be captured again, causing a crash. I fixed this by checking in the battle_control before using an item, if the item is a catching item type if the target is still 'alive', and if not, tell the player that it failed etc. If you can replicate might be worth fixing for everyone etc
Viewing post in Yal's Monster Collector Engine comments
Oh yeah, might need to have a look into that. I know even the released version of Pokémon Colosseum had a bug which let you duplicate items if you used an item as the first party member and then reordered items on the second party member's turn (so you could get infinite Master Balls that way) so it seems this kind of stuff is tricky to get right.