Its at 01:44:20
https://ufile.io/qgosjzdl (testsunkbug.scm map by me) be sure to check this map out.
http://www.staredit.net/sc1db/file/3699/ (by other)
Artosis i think said he manipulated the sunken to do this but this doesnt seem to be true. This happens because sunken has a special attack (doesnt work with photon cannon as you can see in the .scm map by me)
So its quite simple. Leave the sunken alone but it needs to be targeting by itself on an enemy when it either becomes attacked or a structure/unit becomes hit beside the sunken so it re-targets on the attacker.
Each unit in game has an "help ally range" that if a unit beside it becomes shot the range triggers allies to target the attacker for help.
So my understanding is that you CANNOT use this bug to target "infinitely" over the map just because it first needs to be attacking and also get this specific command to re-target for help and that wont happen all-over the map.
Conclusion: The lesson here is.. Dont put pylons infront of your cannons so sunkens can hit em. Then they will be able to hit your cannons.