Remember my request on the mailinglist regarding 'back' button? I've now found what makes users use the 'back' button:

Our tracker has several forms where instead of a 'menu' in html we use a 'field' with a (list) link to a help-popup, similar to what roundups own tracker does for the fields 'nosy list' and 'superseder'. The reason is that there are simply too many values to chose from.
Many of these fields produce a traceback when entering wrong values. In a tracker with 'debug' of the web-interface turned off, all the user sees is 'An Error has occurred'. In all the cases I have seen, an exception from the postgres backend

<class 'psycopg2.errors.InvalidTextRepresentation'>: invalid input syntax for type integer: "xxxx" ..
is seen (when debug is enabled in the web interface).

You can reproduce this in roundup's own tracker by entering "xxxxx" in the superseder field.