OK, not sure where to put this one, maybe the moderators will have to move it?
I'll start off with the one that almost did me in, I'll call it "The Happy Yamaha Owner".
One day a guy came in on his RD350, complaining of a slipping clutch. Or, that's what he said he thought it might be, wasn't sure. I asked him what he meant: he said, "Well, when I get on it, it winds up pretty good before it actually takes off".
Well, OK, that does sound like a slipping clutch.
In those days, I often took these bikes out for a test ride first to either confirm or figure out what the problem really was, and it was a real busy Friday. He left the bike and key, and about half an hour later I donned my helmet (ya never know...) and fired up the 'Hammer to take it up the street. I noticed the rear tire was smooth, so decided not to flog it or get too crazy with the throttle, right up front.
I rolled out onto the blacktop in between the traffic, then grabbed a '2-stroke handful' to test the clutch in 1st gear. I felt the whole bike shudder under me, then it crabbed off sideways to the right and QUICKLY ended up (those of you who ever rode the RD know what I mean) in the ditch at about 30 MPH, with me trying hard to pull it back down, quick! The RD had a lot of extra fuel to burn off after a throttle jump, so it was real 'exciting' for about 10 more seconds until I got it back under me, and stopped. That took a while to stomach.
The rear wheel was pointing off to the right about 20 degrees: the left rear chain adjuster bolt had folded up and snapped off because the owner had a loose rear axle, probably forgot to tighten it after his last adjustment. Dang 350 rocket had almost shot out from under me, would have ended up in the next-door machine shop's front office!
But, the owner was right: something had noticeably slipped under power. Hmmm...I pulled the wheel straight and tightened it up for another go. Got back on the pavement and tried again, expecting the front to at least lift up with this throttle blast: nope. But, as I let off the throttle, I smelled burnt rubber(!). So, looking back, I goosed it again: the rear tire lit up like a dragster's start.
It wasn't the clutch: the tire couldn't hold the RD down! I took it back and installed a high-grip Avon on the back and called the owner, who came the next day for it. I told him all that had happened and charged him for the new chain adjuster, called him a dope (actually, it started with 4 letters, but had more added on) for doing that to me (I was a badass biker sort in those days...). He laughed when I told him it wasn't the clutch, but the tire: he left then, saying, "Yeah, I'm really happy with this bike...".