Or the hosting provider updates to 4.5 without telling you. I had a fun time moving a site from 1.3 to 1.6 a couple of nights ago rather unexpectedly (<- The destabilize runtime bug forced my hand). If only I had finished the version in azure in time
to move it over there.
On a positive note. That is all of my hosting sites now on 4.5. A handful of my modules are running .Net 4.5 as well. I had to update my local copy of Orchard.Contrib.ShapeMessaging to 4.5 as the mocked HttpContext, HttprRquest and Httpresponse didnt conform
anymore (to the new version of mvc and asp.net with 4.5). Thats so far been the only problem i've seen in going up so far.
So the major problems will be those migrating quickly, and some modules that have become a little too old for the framework and .net version?