Today's category was a tough one to answer as "surprising" can mean many things. Surprised by a game's mechanics? Surprised by how great/how rotten it was when first playing it? But I decided on Paranoia -- a game that surprised me by breaking my preconceived notions of what an RPG was.
In my early gaming days, games consisted of going for "the win." We were trying to defeat the evil wizard, rescue the princess, find the treasure hoard, and a host of other cliches. But Paranoia breaks those tropes because -- and this is key -- YOU CANNOT "WIN". EVER. From the start, the odds are horrendously stacked against you and your troupe of troubleshooters. The mission is a sham. Your equipment is shoddy. Your intel is incorrect or misleading. And your teammates all want to kill you. Hell, you'll be lucky to get out of the Briefing Room alive! So Paranoia surprised me by showing me that it's the game and the setting and the journey -- not successful completion of some "goal" -- that makes what we do so much fun. There are no winners or losers -- just "the game".
Oh, and my players were probably surprised by the air horn I hid under the table that was blown whenever Friend Computer had an announcement to make. I'm fairly sure half the table wet themselves the first time that went off, which sets the theme of this game nicely!