It's not an uncommon practice - I experienced that a couple of times in my senior year when I took a few graduate level classes. Are you taking this course because you are really interested in the material? Do you need to take it as a requirement? What grade do you think you'll be able to get? I'm of the opinion that it's totally fine to take a few challenging courses where you might get a B or so for personal enrichment/interest, but if you think there's a risk that you'll get a C or something that could actually impact your GPA I would drop the class - unfortunately that's just the way you have to play the game sometimes.