In the ucp_pm_message_header.html template, there is a <ul class="linklist"> section, which can trigger the "Responsive link lists" section of the forum_fn.js (generating a responsive dropdown menu). This piece is especially twitchy and it happens when the height of the surrounding elements alters slightly.
To reproduce: go to the UCP > PM section (as long as it has buttons like reply/newpm/etc.)
Now alter the size of the buttons using the CSS selector .buttons div a, .dropdown-select.
I have found that if the height of the buttons (or other elements on the same row) is increased by only 2px, the responsive menu will trigger.
Anyone wanting the make the font-size or padding of buttons larger (maybe just by a fraction of 2px) will run into this problem.
Tested in FF 28 and Chrome.