It pretty much common knowledge that the poor little 56 series four speeds aren't all that strong.
I've owned a couple that have gotten a bit noisy after prolonged abuse. (Albeit behind standard A12's)
I've just tonight finished freshening up a four speed that was rolling around in the boot of my sunny when I bought it. It looks as though someone had tried to hook reverse at 80mph. reverse gear on the countershaft, and the reverse idler pinion had been turning into bevel gears. lol
So using parts from a box with a collapsed rear countershaft bearing, (aquired for a 12pack of beers) I fixed it. (Along with 5 new bearings, at a cost of $67)

So now this box should be all fine and dandy, it got me to thinking. since these are widely thought of as not up to the job, what actually breaks? I mean aside from collapsed bearings from a lack of maintenance.
Many of you must have busted a few over the years. What actually broke?