Yup, musicians can be sick, drugged up, hung over, tired, out of practice due to it being the first show of a tour.......... so many factors.
2 smaller bands I use to book regularly that were Hit-n-Miss were DIgital Undergroud & the Young Dubliners. Digital would depend if Shock G was tripping his balls off on some pyschodelic because if he was the show would be more artsy almost with a jazzy feel until he played The Humpty Dance. If he wasn't tripping then the whole show was on a different level with fun energy. The Young Dubliners are a band from Ireland that loves to drink. They drank allot even by Irish standards. Had a few nights where I saw the sun come up while still partying with them after a show. If they showed up for sound check seriously hung over I knew the first half of the show was gonna be pretty dam low key plus Keith's vocals would sound rough. Then by the end of the night when they had drank off their hangovers getting back into the groove they would be rocking and not wanting to get off stage.
I almost listed Soundgarden as the worst concert I have ever seen. They broke up and got back together after 13 years with their first show being the Headline Spot for Lallapalloza in Chicago. They played a private practice concert at The Vic Theater in Chicago on friday night and were scheduled for the closing slot of Lolla on Sunday Night. The show at The Vic was gawd awful to the level so bad that I was embarrassed for them. Was hard to look them in the eye after the show and say that they sounded great. Literally I just said to them something along the lines of "Man it's great to have you guys back rocking the stage". Even their management team after the concert was concerned because of how bad they sounded. Looking back I think part of the problem was they were just a tad bit rusty and the other was the accoustics & sound at the Vic Theater. I think it was a very "Live Room" and their sound engineers hadn't worked with them on any Live Shows. Very poor mix that was too loud with muddied vocals that were grossly overpowered by the instruments. Sound just bounced around in there all night making it hard to understand a word Cornell sung. But I will say that we were all worried about the show at Lolla but they were fire that show. Absolutely amazing show that and one of the better concerts I have ever seen. Had over 100,000 people rocking their balls off. The mix was so much better plus the energy was off the charts from the crowd.