bad LOR do happen, and lots of times, it happens to those who are desperate for letters from professors. Not so much as professors saying " I think this guy should not go to your institution" but more like a very short note, "He was one of my student, but I don't know which one. He got an A- so I guess he's good enough, I wish him the best in med school." If you are really desperate for letter of recommendation, please observe your writer first, kind of ask around if he/she is a helpful person type. On campus, there tend to be a group of professors who write LOR on the regular basis, so these people would know the game well enough. Also, if you use any type of letter service that gives you the option to have non-confidential letters, you might want to use that option for people you have qualms about.