I found this in a small box of antibiotics in the cold room. It's mycophenolic acid that had somehow eaten through the foil ring (blue) that held the bottle's rubber seal in place.
I had no idea why we would have this chemical, but Wikipedia says it's an inhibitor of de novo purine biosynthesis in eukaryote cells (but maybe not in E. coli?), so maybe we had been planning to try it on a protist or on Haemophilus. I threw it out of course.