How sure are you? The game is supposed to prevent you from saving when actions like these are performed. In this case, you should be unable to save until Saemon has left the tavern.
I found another thread saying that this happened when a save was made before Saemon had enough time to leave the tavern, and I'm pretty sure that would have been the case in my game.
This is just a general sort of reply to saving in such a situation: I find it's quite possible to save in many situations you shouldn't be allowed to if you're either quick enough, or if there's a small amount of lag, or (in the case of combat) enemies temporarily go out of view as they chase you or something. Just a few cases that come to mind straight away - any combination of these and other circumstances can allow you to save when you "should be unable to". I'm sure this is nothing new that i'm uncovering though.
The real point is that you don't have to be actively trying to catch the game out and save in such a manner, it's quite easy to do this at least once or twice per game (considering bg is a long game) even when you're experienced with mechanics, nuances, and general behaviours and plot related actions that occur throughout the game, let alone if you're new and want to save before anything you deem "important", "just in case".