Recently, I took a crowded yoga class, where we all had to situate our mats close to each other to make room for everyone. The closeness meant we all had to be aware of our neighbors (especially during Sun Salutations), but the space issue didn't really bother me until we began a series of poses that required more lateral space, starting with Seated Straddle. That's when my extremely flexible neighbor plopped her lower left leg squarely onto my mat.
I'm not a complete germaphobe, but the idea of having a stranger's foot all over my mat grossed me out. I let the moment pass without saying anything, but afterward I wondered if other people would be as annoyed about having to share space in a crowded class, or I was making a big deal over nothing. After all, my neighbor was fine with it — she seemed completely oblivious to the fact that she was interfering with my personal practice!
I tend to avoid crowded classes since I like my space, but other people probably love the energy of a full class. What about you?