We played a large stage at the county fair this past weekend, and something happened I'd never experienced before. Whenever we were playing a tune in E or A, the bass and rhythm guitar were beating badly against each other, and sometimes both would disappear altogether. Everybody on stage heard it and were visibly irritated. The rhythm player and I both immediately questioned our tuning, even though we'd both used the same tuner right before we started. So between songs, he gets the tuner back out and puts a big pause in the action while checking his tuning. Verified good, he took it off and handed it to me with a callous nod toward my bass as if it was the culprit. I clipped it on and quickly proved to him it was dead-nuts accurate, and strongly suggested he get busy playing the next song. (Sheesh, dude! I'm pretty sure I know how to successfully use a clip-on tuner! is what the back of my mind was shouting.) This acoustic nonsense went on for a 70-minute set and was intolerable whenever we hit open A or E strings together. Oddly, tunes in most any other keys seemed just fine. I was still never so glad to leave a stage as I was that day. The audience had seemed content, despite the fact I was certain this had to be equally obnoxious to them. After we got done, I instantly saw what had happened. They had the bass running SVT4 -> SVT810, the rhythm running through an (open-back) mic'd up Twin Reverb...and the Twin was on a rack case lid, about a foot in front of the SVT, and overlapping the lower left hand speaker. Somebody had moved it right before we started playing - I think they were adjusting the kick drum mic and just never put the guitar amp back where it belonged. We were adding/cancelling on open As and Es as those were tuned close to identically, just an octave apart, so about all we'd hear during cancellations was the harmonic misalignment between our instruments...and it was awful! This was all neatly summed out at the board and routed back through our stage monitors, as if to add insult to injury. Of course nobody in the audience heard it that way from the FOH - it just sounded like guitar and bass doing their thing. In fact, the bass player for the next act - an area guy with skills and experience similar to my own - told me as he passed by that it had sounded incredible out front. So the first thing I did was warn him about the conundrum with the amps/mic. He listened in horror and then thanked me vigorously for the information. Once they started playing, I noticed the Twin had been slid about two feet back to the left. Good decision! So, yeah. Don't ever do this. That's all, really. Just a quick laugh at my expense.