Gmail losing email?

Has anyone else had a problem with Gmail simply losing mail?

I registered an application and the author sent me a license key, but I never got it.  A few days later I sent him and email and he sent me a copy of the message he originally sent me, so I searched for the license key and found only this email - so it hadn't ended up in the Spam filter.

Occasionally when I sign up for a forum, I never get the "click this link to verify your email address" message.  I've assued it's just been broken sites, but now I'm thinking Gmail is just dropping mail on me.