Not orchestra bloopers, alas, but a couple Canadian production ones:
Once, Todd Waite, who played Enjolras in the original Canadian cast in 1989, was somehow thrown off the stage and into the audience, where he landed more or less in some woman's lap. They were both fine, but he was so mortified that he came out in costume during the interval to apologize to her.
Also in the original Canadian cast, they were in the habit of singing humorous parodies of the lyrics backstage and one evening, one of the convicts slipped and actually sang one onstage:
"When I get free,
I'll watch TV,
Then I'll dust!"
(This story was relayed to me by original cast member Duff MacDonald, but I don't recall who the convict in question had been, it wasn't Duff himself.)
And lastly, during preview week of the current Toronto production, Mark Uhre's (Enjolras) gun jammed onstage and when he went to pull it back, it cold cocked him in the face and gave him a black eye that lasted for two weeks.