When you leave Server Manager running in Windows Server 2012, a memory leak occurs in the Wmmimgmt.exe process.
This one is annoying because Server Manager opens by default. There’s a tip here to stop that from happening. I haven’t tried the 2008/R2 registry hack yet: Go to HKLMSoftwareMicrosoftServerManager and set the value "DoNotOpenServerManagerAtLogon" to 1. There is also a GPO option.
You can get KB2811660 via Windows Update.