I understand completely, however, being able to quickly see where your mistake is would be very useful. I'll just work around it by posting my message into the reply box on a different forum, and previewing it to see where the error is.

Not going to happen, sorry. The parser validates the code so it can make assumption about the structure of it. Relaxing those restrictions would make the code more complex and possibly bug-prone, just to solve that simple problem. You need to correct the tags anyway, so why wait?

I think that something like this would be helpful. I find the "[/ quote] was found without a matching [ quote]" warnings very useful, but when trying to find the broken tags, it can take a lot of time, especially if you're replying to a large amount of text, or have written a large amount.