HTML Masterpages and UpdatePanels in SP2013

Although this seems to be an old problem (at least from 2010), I just faced this issue a few days ago, so it is still live and kicking in SharePoint 2013.

We had a custom webpart that had an UpdatePanel to asynchronously execute some actions. So far, nothing unusual.

The problem was it worked on a Dev box, but not on the soon to be Live site. Ever heard this one? :)