Credit goes to Chris Hazelton for writing these and Pat Recob for spreading the word.
15 GUIDELINES FOR SITTING IN ON SOMEONE ELSE'S GIG
1. Don't ever go to someone else's gig expecting to sit in. Go to someone else’s gig to listen to them. 2. Don’t ever ask to sit in on someone else’s gig. Wait to be invited. If you can’t compliment what is already going on, decline the invitation. 3. Don't even bring a cased instrument into the building unless you’ve been asked to do so. It comes off as presumptuous. 4. If you’ve been asked to go get your axe, don't take it out of the case until you’ve been invited to the stage. 5. If the bandleader doesn’t know you, someone else in the band or another reputable musician in attendance needs to vouch for your abilities and character to the bandleader. 6. If you’ve been invited to the stage, that invitation is good for one tune and a provisional second. 7. Don't take a solo longer than the folks already on the bandstand are taking. You should have been in attendance long enough to observe this. 8. Don’t be bossy to the other musicians on stage. 9. Don't be wasted. 10. Don't be high. 11. Don’t be looking like a hobo. 12. After you've played, say thank you and put your axe back in its case where it shall remain for the remainder of the night. 13. Don't go fishing for compliments. 14. Don’t post video of you sitting in to Facebook, Youtube, etc. unless the bandleader has given you permission to do so. 15. THIS IS NOT YOUR BANDSTAND. THIS IS NOT YOUR GIG.
And I will add another one: don't be steppin' all over someone else's solo.
There are exceptions to some of these depending on the circumstances.