Some machines have some unfortunate issues built into them - but sure was not intentional.

I have a Iron Man and on occassion when the ball is launched out into the plunger lane some times it doesn't make it and falls back down and then it will try again and some times eject 2 balls starting a mini multiball. I can't speak for other games but my theory with this one is that the balls get magnetised and has caused a few little bugs in loading on the ball guide etc. I cleaned the field (as I am the second owner) and put new balls suited for magnet play and so far so good.

As for balls getting stuck, every very rare now and then a ball gets stuck on the edge and between the legs of the iron monger. When the machine looks for the ball, Iron monger goes down and up and 9 times out of ten clears, but other than that pretty good. One the one time the iron monger gets stuck and does risk gear damage.

I have played Tron and Dark Knight and on both had balls get stuck, though dark knight worse. I also played a AFM and one ball got jammed near the the top right of the centre alien ship and then 2 balls followed and then the game was useless - though I think there was some crap on the playfield.

Overall if it is a home pin, not a drama really, as you quickly take the lockdown bar off,slide the glass down a little and release the ball and generally keep you game clean even better. On a sited machine jams are a pain.