Update: I called Penn and they said that if I think it's not relevant or doesn't add anything, then I don't need to write it.
Slightly more relevant question: Emory asks you to list all of your research experiences that are longer than 8 weeks. I withdrew from a research for credit course during my junior year. To cut a long story short, the person I met with over the summer decided he couldn't have me and passed me along to his friend who had zero funding, zero graduate students, zero postdocs, and basically relied on 2 undergrads, 1 who had no research experience, and me. It was not a good experience in any way. I was also dealing with some health issues at the time and my advisor told me to withdraw from it. Then, she said I wouldn't have to mention it, but in another more recent conversation, she said that some schools ask for all experiences and that I might have to list it.
I checked exactly how long I *worked* in this guy's lab and it was under 8 weeks. Do I have to list it for the Emory/Penn application? I'm obviously not getting a letter from him and have 3 other strong references and will be working for a fourth group as well.
I worked in a similar lab for a matter of a few weeks. I basically sat at a desk and did homework because nobody was around to train me. The PI taught me how to do a few things himself, but I never really did any research. I left pretty quickly. I don't count that as a real lab experience and left it off my application.