Two years now on my Mark Bass LM II never a failure
(but I carry two to every gig)
Basically we carry backup of almost everything ... except PA and monitor cabs (and I make them so I know they will work)
It always seems to be the little items that go, cables and such and we carry spares.
Yamaha EMX 5000 powered board ... Beheringer mini as backup (use a spare LM II to the mains) vocal only
Two bass speaker cabs ... (mine)
Two guitar cabs and heads (Dr. Z 1x12 combos)
Extra mics
Lights are all LED but we carry 4
Two basses ususally a Jazz with a P bass as backup.
Drummer carries spare heads and pedal parts.
Really a spare of some sort to almost everthing, too much to list!

... If it can break IT WILL ... seen it happen to other bands