They've done this with several abilities intentionally (ex warrior defiance and deliverance skills) and unintentionally (elemental wards shared CDs with DoM/DoW skills, yes, you could change to Botanist and have a 60 second cooldown wait for an instant recast skill).

I added Gig to my fishing macro. There's no waits in there, so it does the first one on the list available and all others fail to execute. It's something along the lines of:
/micon Cast
/ac Mooch (I have a 2nd macro without Mooch)
/ac Gig
/ac Hook
/ac Cast