I notice the issue here too, if I remove trailing / from this topic url
http://monox.mono-software.com/Mono/Pages/Discussion/dtopic/70u61PGP2kW3jqc8AV_HZw/Blog-posts-needing-at-their-end-else-get-404/
it gives 404 error (I see no custom 404 page btw, just the browser's 404 in Edge)

Hello,

One solution to this problem is to create 2 rewrite rules in your web.config one with a trailing slash and one without. In your case you need to create a second one without a trailing slash.

Regards

aren't too many rules slowing down the system? my 2nd rule seems to work when trailing / is missing, but doesn't work when it is present. How come? The regex pattern I use there was created from the 1st one and I use a (/...)? so I'd expect it to cover both cases. However it doesn't work with the / and I seem to also need the 1st rule (the original one). Can't I merge the two rules somehow?
  I really don't get the syntax being used there for rewriting urls, think I had read you were using some 3rd-party library instead of the rewrite system Microsoft added later on to ASP.net. Is there any doc for it?

Moreover there are many other rules there related to editing a blog post etc. which I wonder if is needed to also cover in case of missing final /
If those other blog-related urls I saw at the rewrite rules section always have ?... or other params at the end (is this the case?), it is less probable that user would remove the / before the params by accident (since / is not at the very end of the url in that case), so I should be safe with fixing just this one rule

Hi Zoomicon,
MonoX uses a custom module for rewriting, but the syntax is the default syntax used for regex replacement in .NET.

Have you tested your regex, because there are a few cases that might not match as you expect it:
1. ^(.*)/(.*)/post/(.*)/(.*)/(.*)(/(\?(.+))?)?$ - this will match a trailing slash and non-trailing slash, but you won't be able to match query parameters in case of a non-trailing slash URL
2. regex engine is greedy by default which means when your regex is used, the match indices will change depending if the URL has a trailing slash or not - means you should either update the regex to match the slugs correctly, or change the "to" portion of the rewrite rule

See for yourself on http://regexstorm.net/tester

Enter your regex in the pattern field, url in input field and the "to" portion of rewrite URL in the replace matches with... field and then observe the regex matches on the split list tab as you add/remove the trailing slash and/or query string.

Looking at your example, regex
^(.*)/(.*)/post/([^/]*)/([^/]*)/([^/\?]*)\/?(\?(.+)?)?$
and replacement string 
$1/$2.aspx?BlogSlug=$3&BlogPostId=$4&$7

could work as intended, but please test your edge cases as well.

Regards,
Vedran