A friend of mine and I both encountered this last night on Kongregate.

At one point, my friend had 75+ achievements. When he earned another, he went to look at his achievement page, and for some reason, it recorded him having already obtained "Gotta Unlock 'em All" for obtaining 150 achievements. On his stats page, it said he had unlocked 159 achievements, when he definitely had not.

I too checked my own page, and it said I had unlocked 166, when I knew for a fact I only had 83. However, this error corrected itself when each of us refreshed the game, and now it currently displays the proper amount.

My guess is that for some reason, when a particular achievement is earned, it registers a x2 effect that makes it look like we have double. I think my friend had 79 achievements, which when doubled would be 158, and then he earned one more. I am not sure whether the 150 achievement boost is actually applied early or not, so I think this might just be a visual bug.