Seems like it might be an obvious thing to remember, but we all forget from time to time. Having long fingernails really put a damper on flexibility and fluidity when you're playing the cello. As you can see in the left side of the picture above, your skeletal structure has to compensate for those long fingernails and that's bad news for your practice time.
Tip: A lot of my students keep a nail file in a safe place in their cello case. I keep one on my music stand. Really helpful to have around!