When an HTTP URL passed to url-generic-parse-url does not include a port number, and the environment variable http_proxy is set and also does not include a port number, infinite recursion results.  See the following backtrace for an example:

Debugger entered--Lisp error: (error "Variable binding depth exceeds max-specpdl-size")
  url-generic-parse-url("http://proxy")
...
  url-generic-parse-url("http://proxy")
  url-scheme-register-proxy("http")
  url-scheme-get-property("http" default-port)
  url-generic-parse-url("http://proxy")
  url-scheme-register-proxy("http")
  url-scheme-get-property("http" default-port)
  url-generic-parse-url("http://www.some.where/")
  eval((url-generic-parse-url "http://www.some.where/"))

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