ToME: the Tales of Maj'Eyal

Summoner gaining Fire resistance penetration breaks Fire Drake AI, since they refuse to breathe through each other or their hatchlings, so you'll be doing a lot less damage. This makes a lot of mindpower items and mindstars (including the caller's ego!) effectively unusable for summoners, this should not happen for a stat that is supposed to be a positive thing. The summon descriptions even mention it as a thing that carries over to the summon and therefore you want.

One possible solution to this could be just to flag summons as not hitting themselves or allies. It would make a fair few of the effects more usable and less inconvenient, at least (acid fields hitting you/allies on hit when Grand Entrance Hydra is summoned, e.g.) but.