Why is nesting or piggybacking errors within errors bad in general?

To me it seems bad intuitively, but I'm suspicious in that I cannot adequately articulate why it is bad. This may be because it is not in general bad and that it is only bad in specific instances. Why is it detrimental to design error/exception handling in such a way.

The specific instance is that of a REST service. There is a desire by some to use http errors (specifically the 500 response) as a way to indicate any problem with specific instances of a resource. An example of an instance resource in this case would be:

  http://server/ticket/80 # instance
  http://server/ticket # not an instance

So this is the behavior that is being proposed.

• If ticket 80 does not exist return a http response code of 500. Within the body of the error return the "real" error as an additional error code and description.
• If the ticket resource doesn't exist return a response code of 404.