After sorting out that mod_security was triggering on common WordPress functions like posting on a brand spankin' new dedicated server, I had some explaining to do:

Client so, why didn't this issue affect my other site?
Me it isn't what you were doing
it is what you were saying
Client O RLY?
Me mod_security triggers on certain phrases
Client as in, writing about lesbians?
OMG!

(Totally quoted by permission. It was that funny.)