This is more graphic design than illustration/art, but I recently had a client that wanted the front of their building in a flyer I was building for them. They were clear they wanted the sign in the photo. So they sent me a photo, and there's a big tree blocking half of the sign. "Oh well," I think, "it's what they sent me so I'll use it." They get their proof and I get a call from the client, where he's complaining that he can't see his sign. "It's the photo you sent me to use..." I say. "Can't you photoshop out the tree?" he asks.
Apparently people think Photoshop is nothing short of magic, capable of plucking out objects to reveal what is behind them. I had a similar experience with a client who wanted to remove two people (who took up a rather large amount of space in the photo) to show the building behind them.
Who else has funny stories of clueless clients?