What was your biggest unexpected cost in a flip?

[Phillip Cooper]

I understand many have flipped 5+ houses or have tremendous experience with the renovation process. However, as a contractor I'm curious what was the biggest "miss" or unknown finding you've experienced in a flip? Foundation, piping, asbestos, or something many of us cannot fathom. It's easy for those starting out to forget due to their lack of knowledge on certain trades, but curious about those "oh s#!" moments.