To be honest, I've felt that tags are odd for a long time but never bothered to actually check the code and see what it's doing. I'll take a look; it's either broken or the instructions are unclear. Either case should, of course, be fixed.
using <nobr>...</nobr> doesn't work, and if you have working tags around it, it errors out, example:
<nobr>test</nobr> ;I did not use < and > here
Using <tt>...</tt> on the outer tag:
<tt><nobr>test</nobr></tt> causes a red error to appear at the bottom of my post.
<nobr> is not allowed by the HTML filter, which explains why it doesn't work - the tag gets escaped. The issue you described with the error message is separate and appears to affect any combination with disallowed tags.
I'd consider this a bug (because an end tag does exist, it just never gets found), but I'm not sure if it'd be worth it to fix it. It may or may not (haven't looked into the filter's code too much) require reworking parts of the HTML filter and that's something I'd rather not do as it might open a security hole. It's also something that would only happen if you're using disallowed tags, which you shouldn't be in the first place.
Another discussion could be whether we should allow the <nobr> tag. I don't see a reason to considering it's completely non-standard and no browser supports it.
When you edit a comment in a submission, you get redirected to the main page instead to the submission's page. Bug or not, it's kinda weird.
I can be a second witness to this as I encountered it yesterday.
Not particularly bothersome for me personally since I don't leave comments on submissions super frequently (especially not any that I need to edit). But it is a bit strange.