.NET Framework 4.5 is an in-place upgrade for .NET Framework 4.0. What this means is that it's impossible to have both 4.0 and 4.5 installed. If you want to run just a single application on 4.5, you must run all applications written for 4.0 on 4.5 as well. This places some very strong constraints on how compatible such an upgrade must be.
Ever since this was announced, a number of people expressed their concern that the required level of compatibility is not going to be achieved. I'm certainly one of the people who feel an in-place upgrade with so many changes is not such a great idea, to say the least.
Now I'm on 4.5, and I've run into an incompatibility myself. One that makes every single GUI I've built and tested in .NET over the years have a fairly annoying quirk on machines running 4.5. Read on for details.