Yes, there are risks of the sort you describe. The .NET 4.0 interface is unchanged, so if you target .NET 4.0, your program will compile and run on XP with .NET 4.0, but the .NET 4.5 implementation of .NET 4.0 contains bugfixes as well. If you rely on .NET Framework not being buggy, and the only available .NET Framework for XP is buggy, your program is going to experience those bugs. I’ve had this happen with Entity Framework, but there could be other bug fixes as well.