I kept getting locked out of company systems and we eventually tracked it down to our intrusion protection system, OSSEC, blocking me due to these calls on a Jenkins server:
"POST /ajaxExecutors HTTP/1.1" 403
Turns out if you leave a Jenkins tab open in a browser and the session times out, Jenkins goes right on polling for build information with AJAX
(I've tagged core and the Dashboard View Plugin, because this happens on dashboard pages I think, but I'm not 100% on that). The consequence of this is a continuous string of 403 response codes which, after a little while, will trigger any IPS worth its salts to block the IP address - which is exactly what happens to me.
Why Jenkins bug? Because most applications I've come across handle this by redirecting folk to the login page if they get a 403 from an AJAX call, not keeping on hammering on those 403s. If Jenkins did that, there'd be one 403, a redirect to login and there the browser tab would sit, showing a login screen. And an IPS would not be triggered.
Edit: removed the dashboard tag, realised it's definitely the Build Executor div causing this. Also, it's been noted to me this is a regression as the Build Executor box didn't used to do this. Perhaps the response code has changed or the way Jenkins handles session timeouts?
Hope that makes sense!