I think the worst characterization I've read for Renji was in one fic where he goes completely apeshit when he finds out Rukia is having Ichigo's baby and he somehow thinks he has a snowball's change in hell of beating his ass. But then Ruaki jumped in the fight and he kills the baby instead and kills himself in grief.

This one is not nearly as horrible as that, but OMGWTF and Kill It With Fire(tm).