It is possible to have a farm with different front end servers with different OS installed on it , one 2008 R2 and one 2012 ? if it can be done , is there any constraint related to that or mal functionality that may happen or error to occur ?
Should work, we are running 2012 on our App server and 2012R2 on our Frontends no problems with that. But if you are going with 2012R2 I believe SP1 is a requirement.
But if you can get the same os on all machines, less hassle, when applying windows updates and bulding test environments etc.