(I posted this enhancement ticket for Quartz, but I thought it might be a useful feature for Castbars too. I was considering switching from Quartz to another cast bar addon to get this feature, but as far as I can tell, none of the cast bar addons have it.)
When I interrupt a spell the Quartz bar jumps to the end and goes green. However, when I interrupt right near the end of the cast, I find it's difficult to tell whether the cast finished OK or was interrupted.
It would be great if Quartz had a special icon next to the cast bar that showed only when there was a successful interrupt. It would also be good to show a different icon next to the cast bar for a failed interrupt that came in too late.
Sometimes it's clear that an interrupt worked because you can see the casting effect failed in the game. However, other times it's not so clear. For example, Feludius in the Ascendant Council still waves his hands at the end of the cast time for Hydrolance, regardless of whether he was interrupted. Depending on my camera angle and other things going on, it's not always easy to tell whether the blue lines actually left his hands, so I have to check the raid frames to see if someone got hurt.
This is especially important here in Australia, because our increased latency means that interrupts are riskier.
I know that there is a combat log event for successful spell interrupts. There are also a bunch of addons that report interruption success and failure in the chat window. But it's distracting to have to look in the chat window to confirm that an interrupt worked.