You receive this error on your asp.net based site:

Server Error in '/' Application.

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Server Too Busy Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Web.HttpException: Server Too Busy

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.
Stack Trace:

[HttpException (0x80004005): Server Too Busy] System.Web.HttpRuntime.RejectRequestInternal(HttpWorkerRequest wr) +148 Server Error in '/' Application

The term "server" here doesn't mean "the web server is too busy".  Instead, it's referring to the worker process which hosts your ASP.net code.  Each site is configured to utilize it's own application pool/worker process to host it's own code for application isolation and security.  This rare error occurrence usually means something has happened to your worker process (encountered some error it cannot recover from), and it has become unresponsive, and IIS is unable to recycle the application pool to shut down the worker process, and bring up a new, healthy one.   Assuming this is the problem, the solution is usually as simple as notifiying our support team of the condition, and they will manually shut down your worker process... allowing IIS to spin up a new, healthy one. 

Note that this is a different condition than the one discussed here:
ERROR: HTTP Error 500.13 - Server error: Web server is too busy