Machinist - Rapid Fire in GS Bug.

Frequency: 100%
World name: Ultros
Character name: Ato Punto
Class/Level: Machinist / 60

This takes place in Battle.
Steps:
1. Enter Gauss Barrel Mode.
2. Use Rapid Fire
3. Immediately Queue up any ability that would normally have a cast timer in this stance.

Once global cooldown finishes refreshing after using rapid fire is used and the queued ability initiates, it gets stuck in a casting state rather than being instant cast. And it's impossible to cancel the cast, and continues to cast while moving around until the ability fires. This also causes one of the three instant cast weapon skills given by Rapid Fire to be used up despite having not been instant cast in the first place.

That's what server lag is.

If I use swftcast and then immediately go to cast something, half the time it doesn't go instantly and I have to interrupt it and cast again

This issue is un-interruptable.

As I mentioned, once the cast starts, you can run around, jump up and down and what have you but the ability will continue casting. I do not have this issue with swiftcast, that flows flawlessly 100% of the time for me.