Yes, blockchain.info sometimes does that as well.
What happens is there is one transaction spending let's say the existing 0.05 bitcoins. That transaction is written to the blockchain, but is not comfirmed yet. Then the wallet owner makes another transaction, spending 0.05 bitcoins to another address. The second transaction confirms and the bitcoins are spent. However, blockchain sees both transactions, the confirmed one and the unconfirmed one, listing the balance of  - 0.05 bitcoins.
Just a glitch in the system, there is actually no negative balance.