You guys found all of the ones that actually made it into the game. We had art and text for a few achievements that were never implemented in the code, but there are no undiscovered secret achievements. Sorry!
Some of the world builders added secrets that they didn't tell any of us about! Some players stumbled across one a couple of weeks ago -- there was a dead-end street in one of the firebog areas where, after 30 people had entered the street, 3 decorative firebog jars would appear. Once three Glitchen stood underneath each of the 3 jars, the jars would disappear, and 3 actual full firefly jars would drop down.
A few games (e.g. fallen london) use unattainable achievements as locks for new content, so that testers can test 'em but the playerbase is left staring at a teaser of what is to come. Or just to lock it in general, i suppose.
I guess when the skill (BAIII, i think it was) was ready to be released, the impossible requirement would be removed so everyone else could learn it
I really like the design of the badges and hope those unimplemented artwork and writing bits may be revealed somewhere in the future, perhaps in the Glitch Artbook or some other collection of unreleased content publication.
The only known unimplemented achievement is Super-Duper World Trouper, for visiting 5077 different locations, seen in the old Encyclopedia icon: http://i.imgur.com/qIQAF.jpg
Based on the Achievement ID gaps, there are between 42 to 44 entries that may or may not have been planned achievements and unimplemented (including possible entries that were duplicated or deleted).
> You guys found all of the ones that actually made it into the game. We had art and text for a few achievements that were never implemented in the code, but there are no undiscovered secret achievements. Sorry!