I've had lots, but one thing that sticks in my mind was the F2 tank I bought on EBay from a guy in New York. It had a dent in it so he took it to a "pro" to fix and repaint, and to be fair, the paint looked really good.
By the time USPS/Australia Post had finished playing football with the box, there was a hole in it, and a corresponding dent in the side of the tank, and as I was gonna repaint it anyway, I took to it with my 3M strip disc to remove the paint, and bondo.
When I finally stripped all the bondo out, I found 2 holes where the "pro" had drilled into the dent and used self tapping screws to pull it, and only bondo'd over the holes!
The PO hadn't used the tank (he was selling the whiole bike for parts) so it wasn't his fault, but if I'd put gas in it, the gas would eventually have seeped thru the bondo and bubbled up under the paint, or worse.
I brazed the holes up and used it, but it rusted out in less than a year, so I was lucky that a member here, "KGhost" did a trade with me, a really nice OEM paint F3 tank with no rust or dings, for a pair of NOS Boge shocks I had lying around in my garage, from memory. Once again, this site saved my bike!