I am debugging an extension that's running in hgweb under mod_wsgi.  It had
a bug that was raising an exception on every request, but the only
information I could get out of hgweb was "Internal Server Error". 
Everything -- even static files -- simply returned the expansion of
templates/paper/error.tmpl.

The problem with hgweb is that there was no stack trace available for me to
debug my extension.

Here is a rough patch that helps:

"""
--- a/mercurial/hgweb/hgwebdir_mod.py
+++ b/mercurial/hgweb/hgwebdir_mod.py
@@ -125,6 +125,7 @@
         return False
 
     def run_wsgi(self, req):
+        import sys, traceback
         try:
             try:
                 self.refresh()
@@ -159,9 +160,11 @@
                             repo = hg.repository(self.ui, real)
                             return hgweb(repo).run_wsgi(req)
                         except IOError, inst:
+                            traceback.print_exc(file=sys.stderr)
                             msg = inst.strerror
                             raise ErrorResponse(HTTP_SERVER_ERROR, msg)
                         except error.RepoError, inst:
+                            traceback.print_exc(file=sys.stderr)
                             raise ErrorResponse(HTTP_SERVER_ERROR, str(inst))
 
                     # browse subdirectories
@@ -180,6 +183,8 @@
                 return tmpl("notfound", repo=virtual)
 
             except ErrorResponse, err:
+                #sys.stderr.write('hgwebdir: caught ErrorResponse %r\n' % err)
+                traceback.print_exc(file=sys.stderr)
                 req.respond(err, ctype)
                 return tmpl('error', error=err.message or '')
         finally:
"""

Problems with that patch:

  * hgweb_mod.py needs similar treatment
  * multiple stack traces for each exception

...but it's a start.  At the very least, it let me find the problem with my
extension!