Readds bugs from previous versions of Minecraft. By default, this mod does nothing. You'll have to modify the config with the bug(s) you'd like to enable.
The current config includes:
stringDuplicationEnabled
tripwireHookDuplicationEnabled (patched in 26.1)
voidTradingEnabled
CCESuppressionEnabled
redstoneDustRepeaterComparatorIgnoreUpwardsStateUpdateEnabled
comparatorDuplicationEnabled (patched in 26.2-snapshot-2)
cachePoisoningEnabled (patched in 26.1)
If there's anything else you'd like added, make an issue on the github page.