Many times people have logged into the tournament realm and as a result have had items marked as collected on live realms afterwards. The result is usually confusion; the only way to get rid of the undesired settings is manually editing the saved variables - not ideal.

It seems that Blizzard provides a function to check whether the player is currently logged into the Tournament Realm:

• IsOnTournamentRealm()

It would be prudent to completely disable the addon if this returns true, and print a message indicating why it's not working there.

Well, it seems that this API is not reliable at all:

it prints false for me on the tournament realm

Source: Discord

So much for that. Some other ideas:

• Use the output of GetRealmName() and maintain a list of special tournament realms, e.g. AU Mythic Dungeons - not a fan
• Finally implement the UI needed to help people remove those undesired entries - though they'd still need to check the FAQ
• Report as a bug to Blizzard and hope they fix it before the advent of the next century - should probably do that regardless

That's all I can come up with right now. None of these options are particularly appealing, but maybe there's another way?