IIS Application Pool .NET randomly changes

Hi,

Forgive me if this is a dumb question (which I feel like it is) but is it possible that the .NET framework for an IIS App Pool to change randomly from v4.0 to v.2.0? We had one of our websites stop working because the framework changed but anyone who has access to this server claims to not have changed anything. I'm sure ANYTHING can happen with Windows but I want to check if I should rule in the fact that random acts as such can happen. 

Here's the environment that we're running on:

- Windows Server 2008 R2 Standard

- IIS 7.5

- Application runs on .NET 2.0 still

- Server is always up-to-date

Thanks!

James