When adding Tweet via Hootsuite schedule posts the Tweet was schedule without the url. I was trying to add a Medium url (Twitter also used to have /@ in some urls). I guessed the @ symbol was likely the cause of the failure. I did just tiny bit of experimenting and it seems as though Hootsuite removes the link if /@ are included in that order (but more experimenting - or looking at the code used by Hootsuite should be done - I spent less than a minute on it).
Given how popular Medium is I am amazed Hootsuite allows this problem to slip through software testing and use. Maybe Medium recently switched the url or Hootsuite just pushed some new code that broke it.
Luckily you can just remove the @ from the url and then it works (Medium just redirects to the url with the @ in front of the username).
But this is a lame error by Hootsuite. I would imagine it will be fixed quickly - so this post won't make sense anymore (but occasionally big sites fail to do obvious stuff - for example Google still fails to use human readable urls or have even mildly acceptable help).
Related: Coping With Bad Web Page Layouts - Use Urls Not "Click x, Then Click y, Then Click z" Instructions - Bad iTunes Ux and How to Submit a Podcast to iTunes - Poor Web Site User Experience (Ux)