Random 'final job validation failed'

[mattle74]

This is running stratum on Grandcoin. Some blocks that are found get submitted fine (show up in wallet, on MPOS website, etc) but a high percentage of them fail the job finalization in template_registry.py. So, a successful submit looks like this:

2013-12-10 15:21:28,936 INFO template_registry # Yay, share with diff above 100000
2013-12-10 15:21:28,936 INFO template_registry # We found a block candidate! 0000000051ea686560073c9d0730fc16f2ffd7296582fd59d5d85702d8c48a50

And then I'll have some that look like this:

2013-12-10 15:25:45,886 INFO template_registry # Yay, share with diff above 100000
2013-12-10 15:25:45,886 INFO template_registry # We found a block candidate! 0000000042948d330a2becce25945b9abb60c8587682c729763c48fac7f0caa5
2013-12-10 15:25:45,891 ERROR template_registry # Final job validation failed!
2013-12-10 15:25:46,422 ERROR bitcoin_rpc # Problem Submitting block 500 Internal Server Error
Traceback (most recent call last):
File "/home/grandcoin/stratum-mining-litecoin/lib/bitcoin_rpc.py", line 49, in submitblock
resp = (yield self._call('getblocktemplate', [{'mode': 'submit', 'data': block_hex}]))
Error: 500 Internal Server Error

Why would some of them succeed and some fail?