You got it slightly wrong. For The Thing, there is only a few seconds while he does his white circle where he becomes immune to interrupts. The rest of the time you cant still cast it on him, regardless if hes actually casting anything or not.
Is it? I also thought he's uninterruptable when he's doing Massive Gore, but as soon as he starts casting his non-stop Gores you can go back to interrupting him.
Bleh... why is BW so bad at fixing bugs? You fix the bug so Overload is useable by the probe... but don't even spend 30 minutes checking the encounter. Really BW? Really?