The default value for 'Display debug messages' in Site Administration->Development->Debugging changes to whatever the value of the tick box is upon refresh.

Replication steps (from screenshot):

1. Log in as admin
2. Navigate to Site admin > Development > Debugging
3. If the setting "Display debug messages" is unchecked, check it, save settings and click Debugging in the Settings block
4. Uncheck (untick) the "Display debug messages" option
5. Save changes to the form
6. Click on Debugging again in the Settings block
7. Note change to default value