This is good pattern but not implemented correctly. It should rewrite to full correct url. It can be abuse like this:
/140/_/ThisProjectSucksDontUseIt-UseThisOneXXXXXX
https://kallithea-scm.org/repos/kallithea/pull-request/140/_...
https://google.com?you_should_use_bing_instead
https://google.com#you_should_use_bing_instead
Query strings and hashes are a little different, since those may contain information you might not want to scrub.
"Vanity" paths in particular can be rewritten because you often have a "correct" value for a given route.
e.g. https://community.nodebb.org/topic/180/who-is-using-nodebb
You can rewrite the tail end of this URL to whatever you'd like but it will always redirect you back to the correct canonical slug.
This is good pattern but not implemented correctly. It should rewrite to full correct url. It can be abuse like this:
/140/_/ThisProjectSucksDontUseIt-UseThisOneXXXXXX
https://kallithea-scm.org/repos/kallithea/pull-request/140/_...