36/52, years ago. It started on the 10th hole after I got a lost ball and had to run back to the tee to hit again (it was a tournament). That got me all flustered and I played like a big baby for the rest of the round.
I'm going to guess the "shoot good front nine then collapse epically on the back" routine is a bit more common then the other way around.
Largest recent was last fall.... 33-43 for 76. It was my lowest 9 holes ever (actually lowest 11 holes at -3), so in that respect it was great, but from the 11th hole on I shot bogey golf. The swing just went away... suddenly I couldn't get off the tee.