This isn't my most expensive, but it's the funniest:
I had just gotten back into town after a long day of driving (tired, not thinking clearly) with a used disposable camera. I took it to the local pharmacy to get it developed. "Would you like the one-hour development?"
me: "Uhh sure ok"
"Ok, we are close to closing time so we'll get it back to you tomorrow!"
me (drowsy): "huh ok".
The next day I was livid and got a refund
.