I know that this would have happened during the crash in April. MtGox went out and the market moved so fast that Forced Executed positions where FE'd so low that it would have made lenders loose part of the principal.

What actually happened in April was that BFX investor money stepped in and covered those losses.

.....

The market is more mature now than what it was in April and BFX has a lot more internal volume now compared to what they had back then (they are basically doubling every month now) so I think the chances of loosing principle has gone down a lot. But you should beware that it is possible.