Back when Glitch was still a game, the Game of Thrones chat would, among other things, hunt for hidden achievements. I looked for some of these myself by doing some things that people normally wouldn't do a large number of times. Today I've looked through the code to find the hidden achievements and see if there were any we missed.
Unfortunately it looks like we didn't. I compared the list of hidden achievements to TomC's list of achievements and he had all of them except for the ones that were actually unobtainable, and for one called the "The Speedhermit." That was an achievement for getting the all time high score for longest "hi" avoidance run in a particular area. Basically if that achievement was working then people had it - someone had to have the high score. Given that neither TomC nor Sumi had it, I'm pretty sure it simply didn't work. Both of them had the achievements for dodging a "hi" sign for 311 seconds, and Sumi had the achievement for getting the daily record 547 times. The idea you could get the daily record in an area 547 times without once getting the all time record is pretty farfetched.
I was hoping to find something neat here, but there wasn't anything neat to find. I think TomC had already determined by using urls or IDs that another hidden achievement was highly unlikely for those achievements uploaded before the end of 2011, but there could have been more that were undiscovered at the end of the game.
Maybe I can work on the code for Eleven or Children of Ur and get some more interesting hidden achievements in there one day.