Had a couple of magneto failures in Bell 47's - just lost a bit of power. Both times I was light enough that it was a non-issue.
Had a bad roll hydraulic cylinder, again on a Bell 47 (TomCat Mk5A to be specific) - causing hardovers with any roll input. As a sub 50 hour PPL at the time with a couple hours in type & no real checkout, systems or emergency procedures training from the owner (took it around the pattern a couple times & was turned loose, maybe 15 minutes total) that was interesting.
I've had the pitot tube get plugged by bugs, so no airspeed indication.
Also have had a rotor tach generator failure, so no rotor rpm indication - still had the engine needle working.
All minor stuff, I was definitely luckier than some other pilots working for one operator - one guy had a centrifugal clutch failure in a Bell 47, on the next flight after I got out. He was in a turn (ag spraying) when it let go, walked away from it but totalled the ship. Another year a Hiller 12E lost the torsional coupler between the engine & transmission right after liftoff - minor damage to the ship (bent gear) but the engine oversped badly & added to the expense. I'd been flying it that morning, he'd been going a few hours that afternoon...