With GTK, Glib and a few libraries/toolkits sending warnings at widget level, we recently saw people being unable to log in, or getting their system crawl. In Fedora a user reported his .xsession-errors had gotten larger than 500GB. Most of the information in that file was spammed by a single process and the output was irrelevant to the user himself.
In the upcoming MDM 1.8, the session output is limited to 200KB (between 2000 and 4000 lines of logs). This limit is enabled by default and the user can disable it in the MDM preferences. When .xsession-errors reaches 200KB, a footer is appended by MDM explaining that the limit was reached and how to disable it.
Another option coming in the MDM preferences allows the user to filter the session output. This option, which is disabled by default, prevents warnings and errors from GTK, Glib, Gio, Gobject, Glade etc… from getting into .xsession-errors.