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While playing a concert, a parade, etc?
Over the years, there have been some doozies!
I'll start it off:
While waiting to step off a parade in central Pennsylvania, real farm country, the "float" that pulled out just before my unit was an antique manure spreader, all restored to its former glory! That was not the best part: on the float, complete with banners, was a politician running for county commissioner! Never have we laughed so hard!
GO.

Had an antique truck's brakes fail behind us on a mild downward slope.

Had to get out of the way pretty quick as it happened, but the wonderful part is enough larger folk on curb watching parade jumped out to grab it and slow it down that it got redirected off the parade route without incident. I even got a hand in on it briefly to help, until it was obvious i weren't needed. We reformed afterwards and finished off the parade.

There was also a centenial parade where i was playing on a trolley, and tractor rig (pulling a float i think, wasn't paying too close attention) let us by as we were ahead of them, and then pull into line behind us. Well, a couple turns in we noticed no one was behind us. Apparently they stalled, and we finished off the parade before they could restart. Which was really weird.

Does that count?

The sousaphone and its player next to me were tackled on the sideline of a Monday night football game just before halftime. It was the only time I was on national television.

Not funny, but certainly strange:
An acrobat from the Cirque du Soleil crashing into the front row of seats during a Pops rehearsal.
The seats didn't give much. Their fails are as impressive as their acts!