The only way you can roll back your forest functional level is from Windows Server 2008 R2 to Windows Server 2008 if Active directory Recycle Bin is not enabled. Review the following guidelines that apply to raising the domain or forest functional levels:
can you provide some example scenarios why someone would need to roll back their forest functional level?
It is very unlikely that you will need to do this, provided you have done enough planning. However in case you need to introduce another old version of domain or if any applications might have unexpected behaviour, you might need to do that.