A timer or a number of "strikes" would be a good idea. Like, if you lag out 3 times then you can't come back, and if an activity is completed your strikes refresh (each part of a raid) I personally haven't tried to use guided games, but I can just imagine how irritating it must be to go back to the start and earn nothing for the same amount of effort. Lag outs and error codes should be detected by the system so crap like this doesn't happen, or at least let them join back, I mean, if I'm there to help people through the raid, even if I don't get anything, at least you kept your oath. Your oath should only be broken if you aren't there to finish the activity, but if you were there after lagging out (and rejoining) and you finished, you're fine, but if you left before the activity was completed, and then completed, is when it should take effect.