Annoyance: Formatting/URL mistakes suppress preview completely Answered
A new feature deployed recently checks the URLs we put inside comments (i.e., the first token inside square brackets) and reports if we haven't given a valid URL.
The annoying bug is that the whole preview is suppressed, and there is no indication of where in the text the error might be. Briefly, the preview actually showed the whole text, but with the error part called out in big red box. That was better, and would be nice to get back.
Failing that, maybe just putting the malformed URL into the preview as a literal string, highlighted or underlined or something might be helpful.