Badges?! We don't need no stinking badges!
I remember picking up HL2 at a second hand store mere weeks after it was released. I already had a steam account for HL and DoD which I reluctantly joined once Valve killed WoN.
Anyway, 2nd hand HL2... I know what you're thinking:
The key was already activated, right?
Well for 5 bucks I figured I'd take a shot give it a go anyway.
I typed the activation key and got an error. It was so long ago now I can't remember what it said. But a search at the time led me to a support page that pointed out the activation codes on the back of the cd sleeves where too long.
I entered the key again, leaving out the last group of letters and numbers, and Hey Presto! New Half Life 2 activated!
It was a rare win for me.