Looks like it was being exploited on Bytecoin
For example, these 2 transactions spend the output 26e8958fc2b227b045c3f489f2ef98f0d5dfac05d3c63339b13802886d53fc05 twice!
Notice how the key image differs only in last 2 digits! That's because it's forged - ie 2 slighthly different key images for 1 output, allowing you to spend it twice by tricking the pre-patch node into thinking that both key images are valid for the given output. There can really be only 1 which can be checked only with a patched node, and one of those TX-es would become invalid.
Note that it's later patched in the code, but the patch applies only after Bytecoin block 1267000, so those double-spend transactions are still in the Bytecoin blockchain and have artificially inflated the supply.
Note that while I was writing this, there were some problems with chainradar block explorer and those transactions disappeared for a while! That doesn't mean that they're not in the blockchain (really, the only way to be sure is to run a Bytecoin node and check yourself if they're still there), but only that chainradar was not showing them (seem to be back now). Just in case, they can also be seen here: