Related-key attacks (why we have XXTEA) are fun to read about (get 4o to explain them to you); for something like TEA, where the key schedule is really simple, the attack is similar to differential cryptanalysis but much simpler. A fun thing about TEA is that was (as far as I can tell) used in anger in exactly one place --- the Xbox --- and blew the platform up. It was used as a hash, and every TEA key has an equivalent alter-ego key or something, I forget the details.
> It was used as a hash, and every TEA key has an equivalent alter-ego key or something, I forget the details.
It's that flipping bit 16 and 31 of a 32-bit word makes the end TEA state the same [1].