When using a drawing tool with a guide, you are also able to edit the guide's horizon line and vanishing points. This may be a problem if you need to draw close to the horizon line or to your vanishing points, as you may accidentally displace them instead. Once you're sure that your guide is properly set up, it is recommended to lock your guides before you start drawing, so you don't have to worry about editing them by accident.
In the Guides view, activate the Lock Guide button.
NOTE The Lock Guide option locks the editing of all guides, not just the selected one. When creating a new guide, the Lock Guide option is automatically disabled to allow the new guide to be edited immediately.