If you are in "paint through" mode, paint will traverse all the way to the back of the model, which can be destructive if you are not aware that the mode is turned on.

To always be aware when you are in "paint through" mode, you can turn on an optional warning outline around the viewport, 
This setting is off by default and needs to be turned on via the Mari Preferences (Edit/Preferences, within the Painting Tab)

You can activate the warning via the Preferences. It is off by default

Mari allows for many ways to prevent paint from being accidentally applied to protected areas. Users can lock channels, objects, selection groups and more.
Unfortunately this can lead to situations where parts of a mesh do not accept projected paint without it being obvious to the user why.

Extension Pack adds a Warning Notification to the Viewport if you are trying to project on an object that has parts of it locked.

When baking the contents of the Paint Buffer, Extension Pack first checks if ..

•No Object is selected

•Current Object locked

•Current Object contains Selection Groups that are locked

•Current Object contains UDIMs that are locked

•Current Object contains Layers locked

•Current Object contains Channel that are locked

If any of those conditions are met, a notification will appear allowing you to Unlock whatever is locked

If you do not wish to see the notification, you can pause the notifications for the duration of your Mari session.  It will then be re-enabled after a Mari restart