It is possible to lose principal on early payments. Worst case, an A1 $25 loan pays $0.13 interest the first month. So, with a $0.25 fee, you lose $0.12, which works out to a roughly -5.5% APR ...
This is one area where LC could use a rule change, to reduce the fees on early payoffs. I think, at a minimum, LC should make the fees no greater than the interest paid, so investors are at least guaranteed to break even on one-month payoffs.