I am aware that for comments we have maximum of 600 characters to type and by curiosity I just happened to type some random words in a comment box. I expected that when comment exceeded 600 characters I will not be able to type but instead it showed - 'you exceeded X characters'. I am just curious but afraid to do this - what happens if I post a comment with more than 600 characters?
In a way literally nothing happens. You click "Add Comment" and the situation stays unchanged. The "too long by" flickers briefly, but that's it.
As explained in a comment by amWhy there is a check on submission.
I dimly recall that in the very early days this was handled much less gracefully, and the text in the comment box got cleared instead.