If you aren't careful, you'll get airborne fungus / mold in your plates or slants. All of the unused slants I made are free from contamination. But one of the plates I made contracted a disease.
This plate had some odd craters in it, I guess from a bubble that was in there. I opened it up and looked at the craters under my inspection scope when it was a day old; I only looked at it for about 20 seconds. In that time, I guess something dropped out of the air. Here the plate is, about 10 days old.