When doing some exchange powershell you would probably get this error or somthing like it sooner or later:

Connecting to remote server failed with the following error message : The WS-Management service cannot process the request. This user is allowed a max
imum number of 18 concurrent shells, which has been exceeded. Close existing shells or raise the quota for this user. For more information, see the about_Remote_Trouble
shooting Help topic.
    + CategoryInfo          : OpenError: (System.Manageme….RemoteRunspace:RemoteRunspace) [], PSRemotingTransportException
    + FullyQualifiedErrorId : PSSessionOpenFailed

Workaround for this is to recycle the MSExchangePowerShellAppPool Application Pool