On PC (perhaps other platforms, but it may be specific to right-click-to-activate) the event to return the cannons to "inactive" status is not triggered when activating another cannon while one cannon is still activated. I happened on this accidentally and was able to repeat it until all cannons reachable from one spot were broken and providing the error "Unable to use the cannon as it is in use by another player" despite being the only person left in the instance.
Time: Always available
Frequency: Always available
World name: Balmung
Character name: Any character (Pentiki Mahougami reporting)
NPC name: XLIII Mini Cannon
Monster name: N/A
Class/Level: 50
Party or solo: Party
In-game time: Always available
Area and coordinates: Castrum Meridianum
Steps:
This is easiest to reproduce at the first battery of cannons which you come to after defeating the magiteck vanguard boss (boss 2 in CM).
1. Approach any of the 4 cannons and stand between any 2 cannons.
2. Activate one of the cannons by right clicking on the cannon.
3. While the pet bar shows that you can use the CannonFire ability, right click on the other nearby cannon (do not press escape or otherwise disengage from the first cannon)
4. At this point, the first cannon will now be "bugged" and will always provide the error that someone else is already using it.

Any two cannons which can both be reached without moving can be broken in this manner.

***Not a hardware or connection related issue***