What is the key caution about basic-triliteral maṣdars?

Noun Patterns: Jāmid, Maṣdar, Mushtaq and 50 Basic-Triliteral Maṣdar Entries

• Patterns vary with attested usage; one template does not fit each verb class
• Every maṣdar must be faʿl
• No maṣdar can ever contain added letters
• A maṣdar is always a past verb