I made a buffer way back in June that I didn’t need again until last week. It had been at the back of the fridge for 4 months, without being moved at all, and LOOK what happened! I created accidental crystals!
I pipetted off all the liquid from the crystal before I took the pictures. The bottle was almost full before I did that.
The buffer was a monobasic sodium-phosphate buffer (pH 7), so the crystal was probably NaH2PO4 with water molecules in there somewhere. (The stuff I used to make the solution in the first place was hydrogenated too.)
The crystal is gone now, though. We’re moving the lab to a new building, so everything has to be cleaned up, and “unnecessary” things are thrown out. Besides, I don’t think the movers would have appreciated the delicacy of my crystal.