When you go to a screen that uses a translucent layer and you try to turn it off, ZC for no apparent reason will take pictures of that screen and take up a lot of memory. This happened to me on betas 178 and 179 (both on the 178 Test Quest). If you are quick enough, immediately save your game and focus on deleting the few (hundred) snapshots that is 100% useless. And no, this is not my keyboard that does this bug, it's a bug concerning ZC.
What, I didn't need all those pictures!
You hit the F1 key. I had set that from doing VSync to snapshots (so I could get a running series of pictures useful for creating animated GIFs of new features, such as the slash attack). I've now set F1 back to VSync and Ctrl-F1 to the constant snapshot effect.