An orphan block is a block that was generated by your wallet, but rejected from the blockchain; any coins created within it are lost. Orphan blocks will happen from time to time on just about all blockchain’s including Bitcoin. An orphan block is encountered when two wallets stake the ‘next block’ for the blockchain at nearly the same time. Since there can only be only be one ‘next block’, one of those closely minted blocks gets deemed an orphan and forgotten.
The first thing your wallet does after staking a block, is send out a message to all the connected nodes(wallets) to let them know that ‘I staked the next block’. If those wallets have already received that same ‘I staked the next block’ message from a different wallet, they will reply with something to the manner of ‘too late, keep trying’. Your wallet will then mark the block it created as an orphan block.
If you are using the NavCoin Core wallet, you can identify orphan stakes in the Transactions tab:
This article is a excerpt from Bocyaj’s article on staking which can be found here.