Blew my freehub muscling up a steep incline. This is why I ride with half of my workshop in my backpack. Had zip ties packed up specifically incase of this happening.
I once snapped a chain and had to walk my bike back home. I never wanted to do that again so I've been pretty much prepaired for anything ever since. Usually it's just flat tire I've had to deal with. Had to fix the chain another time too but had a chain tool and a masterlink that time. So far the greatest breakdown has been when I snapped my derailleur hanger and had to shorten the chain and convert it into single speed to ride home. The freehub had been experiencing skipping for some time now so I can't say I was too surprised when this happened.
Stop dropping watt bombs on cliff faces.
I tend to prioritise the drivetrain for upgrades/maintenance, or go single speed ;)
This hub was warning me about the impeding failure for a good year and a half and I kept ignoring it knowing full well that this day was coming. I knew I'd be able to ride back home anyway so I don't worry about it.
I'm the most atypical mountain biker you'll encounter and preventative maintenance isn't really my cup of tea. I ride it till it fails and then I replace it. I'm actually surprised my rear tire held up for longer than the hub. That thing is held together with duct tape and holy spirit.