This is happening in your own wallet, right? You staked an orphaned block, which happens frequently on all blockchains. Essentially, two clients staked a block at roughly the same time, and your block had less support so was rejected.

Fear not, the main chain never knew you staked that block, so you lose nothing and will be paid out for research in full when you next stake a block on the main chain.

In shorter terms, both your stakes were on forks, which have low difficulty and thus make it easy to stake. =)