i2p before 2.3.0 (Java) allows de-anonymizing the public IPv4 and IPv6 addresses of i2p hidden services (aka eepsites) via a correlation attack across the IPv4 and IPv6 addresses that occurs when a tunneled, replayed message has a behavior discrepancy (it may be dropped, or may result in a Wrong Destination response). An attack would take days to complete.
The product behaves differently or sends different responses under different circumstances in a way that is observable to an unauthorized actor, which exposes security-relevant information about the state of the product, such as whether a particular operation was successful or not.