Mistake #1: Using the broiler. The broiler is a dragon’s breath. It will burn your bread in 30 seconds. Only use it if you like eating charcoal and crying.
Mistake #2: Overstuffing. A sub so fat it won’t close is a logistical nightmare. The heat won’t reach the middle. Keep it reasonable.
Mistake #3: Forgetting the timer. I have done this. I now serve “artisanal dark toast” subs. Don’t be me. Set that timer.