We have recently found an important problem when deleting unused templates.
If you save and reload after deleting unused templates, the per Device Type template references in all recorded data will be out of sync and point to the wrong template or even cause a crash. This is new to v8 but is fixed in the upcoming v8.0.1.
In the meantime, avoid deleting unused templates.
Note: If you are running multiple systems together, synchronization is also affected.