When running CM dungeon, the cannons at the Y intersection do not always for me and a few others I have ran across.
I hold R2 for commands, press circle to start cannon fire, line up the purple mark till it turns red and is under the enemy, and no button pressing after that usually will fire. I sometimes get a shot or two off when pressing X even though no command is there and sometimes get fire from pressing circle. I have this issue on the PS3 and have seen a few on Computer.