Not washing mushrooms.
Mareya Ibrahim, founder of Eat Cleaner food wash products, says most bagged lettuce is washed in a chlorine bath, but she recommends rinsing it anyway "as a preventative precaution." Without question, though, mushrooms should be thoroughly cleaned before cooking, for two reasons. Firstly, they're usually grown in soil using fertilizers and may carry bacteria like E. coli. Secondly, the handling and transportation of the mushrooms can contaminate these little sponges even further. Use wipes or a damp paper towel or kitchen towel to clean them.