Looks like it was being exploited on Bytecoin
For example, these 2 transactions spend the output 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.
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 chain and have artificially inflated the supply.