In any tracker, create lib directory and file "quirky.py" containing text:

class Test:
    pass

then add "import quirky" as the first line of schema.py

roundup-admin should behave as normal, the mail gateway should behave as
normal, but the web interface now throws an error on every access.

Which error message?

Cut and paste from the traceback in the web page: 

ImportError: No module named quirky	

A problem occurred while running a Python script. Here is the sequence
of function calls leading up to the error, with the most recent
(innermost) call first. The exception attributes are:

/mnt/infomaster/roundupdev/schema.py in ()
    2 import time, random
    3 from roundup import hyperdb
    4 from email.Utils import formataddr
    5 from roundup.i18n import _ 
    6 from roundup.mailer import Mailer, MessageSendError, encode_quopri, \

/usr/lib/python2.6/site-packages/roundup/instance.py in
open(self=<roundup.instance.Tracker instance>, name='admin')
  116         if self.optimize:
  117             # execute preloaded schema object
  118             exec(self.schema, vars)
      self = <roundup.instance.Tracker instance>, global schema =
undefined, vars = {'Boolean': <class 'roundup.hyperdb.Boolean'>,
'Class': <class roundup.backends.back_mysql.Class>, 'Date': <class
'roundup.hyperdb.Date'>, 'FileClass': <class
roundup.backends.back_mysql.FileClass>, 'Interval': <class
'roundup.hyperdb.Interval'>, 'IssueClass': <class
roundup.backends.back_mysql.IssueClass>, 'Link': <class
'roundup.hyperdb.Link'>, 'Multilink': <class
'roundup.hyperdb.Multilink'>, 'Number': <class
'roundup.hyperdb.Number'>, 'Password': <class
'roundup.hyperdb.Password'>, ...}
  119             if callable (self.schema_hook):
  120                 self.schema_hook(**vars)

Note that the above error actually blots out line 1 of schema.py, which
is "import quirky". This may imply that the web traceback needs a little
work too.

Potential workaround follows, but the real fix will be to make the
availability of code in the lib directory consistent.

Put this at the top of a schema.py which depends on code in the lib
directory: 

import os.path
import sys

libdir = os.path.join(db.config.TRACKER_HOME, 'lib')
if libdir not in sys.path :
    sys.path.insert(0,libdir)

import quirky

Fixed in r4554, thanks for reporting!
looks like I was using your workaround for years :-)