In Ruby method names ending with an exclamation mark are considered “dangerous” as they change the object on which the method is invoked.
I ran into a problem using one of these dangerous methods last night. I wanted to ensure that strings that I was handling were uppercase. There are two methods in Ruby that do this: upcase and upcase! I attempted to use the latter option. It worked on my first tests, but it failed when I passed a string that was already in uppercase and the string was changed to nil (empty string). This behaviour is explicitly documented, but I had missed that detail. Using the regular method, s=s.upcase works as I required, so it was a simple fix. However, this is an annoying difference in behaviour between the two method and seems to be counter- intuitive.
