Weapons are strictly forbidden on the Blood Bowl pitch. As far as most teams are concerned, that rule has an implicit condition attached of "unless you get away with it". Some teams are more adept, and enthusiastic, about sneaking weapons onto the pitch, than others. Dwarfs, who believe they have the right to interpret the laws of the game however they wish, are frequent offenders, as well as Goblins, who just seem to do it for fun.
Most secret weapons are not, in fact, all that secret. Even if you manage to smuggle a chainsaw or blunderbuss onto the pitch past the gaze of the referee, it becomes pretty obvious once you start using them. Players who use these weapons tend not to be concerned about being sent off at the end of a drive, so much as using the weapons to inflict damage while the drive is in progress.
Some players, on the other hand, limiting themselves to concealed stiletto blades, knuckledusters and the like, are rather more subtle about it, and more likely to get away with it. Some teams have attempted to circumvent the problems brought about by referees sending off their secret weapons with bribes, with mixed results.