Everytime I see an /o on a RE, it's usually the wrong thing. And it irks me to no end when it's used on REs that have no variables in them. It's a maintenance nightmare waiting to happen.

Imagine a RE with /o and no variables buried in a module that's part of a much larger application. In the future, some enterprising programmer decides that the RE would be better off with a $variable than what's already there but he forgets to remove the /o (or it may be so far removed from where he added the $variable, that it's not immediately visible (/x is both blessing and curse :)). Everything will work fine as long as the routine that uses that RE is called exactly once per execution of the program. The moment you put it in a loop surprising things may happen. But you may not even notice anything is wrong for a while. And tracking it down to that one little /o modifier could take ages!

BTW, something like the above actually happened to me several years ago, but I was both the person who created the RE and the future enterprising programmer. :-)