Users are warned of a critical issue when transferring cryptocurrencies between Aptos and Sui networks. If Aptos tokens are mistakenly sent to a Sui address, the funds become irretrievable. This is due to the fact that a Sui address's private key cannot derive the same address on the Aptos network, and vice versa. Despite both addresses appearing valid with a 0x prefix and 66 characters, they are incompatible, leading to permanent loss of funds if transferred incorrectly.