I think it was mostly oss (maybe with local changes, re: why not download a fresh copy?), but there was some proprietary code in there (which may or may not have been intentional), which is what tripped him up.
Right, but aren't we sort of underestimating GS's ability to tag oss with just about anything proprietary-ish, then, when one of your top programmers quits to join (or co-found) a firm in a competing industry (H-FT), maybe hold it over their shoulders. Considering the scope of this and the dollars to be made, Mr Aleynikov should have been aware of his former firm's contractual small print and should not have copied anything without an IP lawyer present.