I generally don't bad-mouth previous employers, as it risks limiting your career. However, the only review I ever left on Glassdoor was of the most toxic software job I ever had. The company basically had nothing but dozens of 1 and 2 star reviews. A few months later I checked again and there were about 10 brand new vague 5-star reviews that were all submitted within a day of each other. One of them even said right in the review something like "boss told us all to write 5 star reviews, so here's mine: This place is horrible."

Just checked the company again, and my review has since been removed (but some other 1 star reviews are still there).