Dad calmly and empathetically replied, "Oh dear, son, that's such a bummer. I know you were working hard on that last night. I'm on my way to work right now. "
"Dad, please, you GOTTA help me out! I really need it!" his son pressed on.
"Hmmm... this is going to cost me a lot of time but it seems important."
Dad then looked at the clock in his car, turned around and went home to get the forgotten project. He then drove to school and looked at the clock again as he handed his son the assignment. 20 minutes, he noted.
He said, "Here you go. Have a nice day!" No lecturing, no reminding how thoughtless he'd been or anything. Just a nice, sweet farewell with a smile.
Dad then had a pleasant day at work.
A few days later it was Saturday. Dad had delayed the consequence perfectly. He said to his son, "Son, remember how I brought you that assignment this week?" "Yep." "Well, that took 20 minutes out of my schedule and I'd like you to pay back the energy that was drained. I really need my car washed inside and out. That will take at least that."
His son went ahead and cleaned the car. It seemed like an even deal in his mind.
Why no fuss from his son? Because Dad had stopped lecturing and let his son's poor decision do the work. The Energy Drain and Delayed Consequences worked in everyone's favor.
Awesome job, Dad!