It's rare, but over the years, I've had one head go south during a rehearsal and two act up on gigs.
A Traynor YBA-1 w/ microphonic tubes acting up during a gig. Limped through the night w/ it, as I didn't then carry back up.
An Ampeg SVT 350T was silenced during rehearsal by an FX loop jack failure (a relatively common failure--and an easy work-around, as it turns out). Switched to a backup Peavey head.
A new-to-me (but well used) Mesa Walkabout got crackly during a NYE gig. I switched to my back-up LMII. Sent the WA to Mesa, and they repaired a dodgy connection to the board.
Funny thing: not long after getting the repaired Walkabout back from Mesa service, I played it on a sub gig with a wedding band. The band leader was really keen to take a DI signal straight from the bass, even though the WalkAbout has a nice DI. I asked what gives, and he explained that he'd gigged over the previous two weeks with two different bassists whose heads had ghosted during the gig (both friends of mine: one whose brand new GK rig died, and another who spilled a drink he'd left on his Euphonic Audio head
).
I've been playing since the 1970s, and over that time I've seen way more guitar amps bite the dust--including one at a gig just last month.
Bass gear tends to be pretty robust, and buying nice gear and taking care of it helps our odds considerably. Still, there's a reason every amp builder keeps a service department. Play enough gigs, and the gremlins will probably catch you at least once.
