I don't have the paper rules here as a reference, but I grew up playing Rummikub with the rule that, if you replaced a joker on the table you must immediately replay the joker in a run or set with at least 2 tiles from your hand. This rule could make the endgame particularly interesting. We also played that the joker froze whatever group of tiles it was in--they couldn't be manipulated without replacing the joker. However, I just looked up the rules online to settle a dispute about making the initial meld, and noticed that both of these special joker rules are missing.
I'm pretty sure that way back when they were official, not house rules; were they? If so, when (and for bonus points, why) were they changed?