Log in as admin Go to site administration > development > debugging and set "Debug messages" to "DEVELOPER" Create a course Create a group in the course Course settings cog > more > Users tab > groups Create a forum activity Under "Common module settings" set the "Group mode" to "Visible groups" View the forum At the top of the page (above the "Add a new discussion topic button") change the Visible groups selector from "All participants" to the group you created Add a new discussion (this creates a discussion in that group) CONFIRM that the group name shows up in the "Group" column for the newly created discussion Change the "Visible groups" selector at the top of the page to "All participants" Go to the groups page for the course Delete the group you created View the forum CONFIRM that you do not see the "Undefined offset" debugging error at the top of the page (as shown in the screenshot attached to this issue) Edit the forum settings Change the "Group mode" to "No groups" under the "Common module settings" section View the forum CONFIRM that you don't see the "Undefined offset" debugging error