Ignoring the intentionally different balls like TZ’s Powerball or Viper’s Glo-Balls, I’ve never seen or heard of a case where a regular steel ball was so damaged that it would roll irregularly. Steel balls can get pitted and corroded, and if balls like that are kept in a machine long term, they can horribly chew up the clearcoat and painted/screened wood of the playfield surface, which could eventually affect the motion of balls. (You can see a related condition on older games where the ball is launched in such a way that it arcs right to left to right at the top of the playfield, following basically the exact same path every ball… over a long time, this can actually carve a groove into the playfield wood.)
But a case of the steel ball itself being misshapen to the point of not rolling in the expected way is pretty much unheard of IMHO.
The only thing that does happen is balls getting magnetised, if the wrong balls are used in games with magnets. Generally all balls have the same chance of becoming magnetised, but once magnetised, act differently.
I’ve never noticed a difference between balls in a machine. Playfield differences are common. Extreme wear can leave grooves, caused raised inserts to have a lip that deflects the ball, cup thin round inserts so the ball rolls strangely or even gets stuck. Even cleaning a playfield changes play a lot, as does different kinds of rubber used on flippers & slings.
@joe, @gammagoat, @YeOldPinPlayer, those points about balls getting magnetized and playfields changing over time are great. I hadn’t thought of anything like that. I can use these ideas. Thanks.