That actually happened to me. Friends of mine run a business I did some low level setting up of IT and design work for. Recommended they use Dropbox for important data in a folder I shared with them. One day, I keep getting notifications from Dropbox. Temporarily disable it on my machine because it annoyed the heck out of me. A few hour later, I get the "I think we have a virus" call.

Turns out that me being annoyed means I now have a fully intact version of the data and nobody had to pay anything.