tl;dr checkwallet said I had 754.5 mismatched ORB, but repairwallet didn't change the balance.
It doesn't affect balance if it's correct, though it removes failed transactions from wallet.dat at least.
I think my wallet must be haunted then.
2111 ORB was 'hidden' by a failed transaction from 2013 or early 2014 (recovered by repairwallet in late 2014), and since then it looks like all that has happened is that I have staked 5 times. The only -1 confirmation listings are orphans from PoW mining years ago. The last PoS stake was 40+ days ago. I'm not sure whether "2 mismatched spent coins" means two unspent outputs, but no two get anywhere near 754.5 when added. Apart from the recent stakes, this wallet should have been virtually untouched since its repair 6 months ago. I have no idea where that 754.5 ORB figure came from, or how the coins came to be mismatched. This is not the first time that checkwallet has reported an error. 
