When returning to the Jenkins tab on my browser, I will often click on the arrow on the right side to schedule a build.  Usually, it says "Build scheduled" and the build is actually scheduled.  Sometimes, the "Build scheduled" message appears but the build is not actually scheduled.

It apparently happens when I return to the tab after having been away from the tab for too long.

Digging deeper when it does not work, with Developer Tools, I can see that there was an error on response to the AJAX call.

Error 403 No valid crumb was included in the request

Bug 1:  The UI will indicate that the build is scheduled when it is not.  The clicking of the button should detect the return value of the request and change the message if the request failed.

I do not know exactly what a crumb is in this framework.  I can reload the page without having to log in again, so I still have a valid session.  A "crumb" is obviously some implementation detail that is leaking to the user, when it is not otherwise known to the user.

Bug 2: The request to start a build should rely on the session, not on some crumb concept that is not known to the user.