From Bug #950645, here's one somewhat reliable way to trigger this: after login, have some windows open (I have Chromium, Gnome Terminal and Transmission set to start on login, don't know if this works if you start them manually), then use the workspace switcher. This causes the launcher to stick (i.e. autohide to fail).
Interestingly, if I then killall unity-2d-shell, even this recipe then fails to trigger the issue until the next login. But it seems 100% reproducible right after login.
(I'm just hoping that #950645 really is a duplicate of this one. For me the trigger's always been something else in the launcher since I almost never use the workspace switcher.)