Had a second dumb thing happen. Person to guess will get 6 flies from me. One of my patterns, by mutual decision.
Rick
I’ll let someone else win this time but I am sure that it is something in my vast repertoire of dumb moves. The worst thing isn’t making a mistake, it’s repeating it a couple of years later.
You got home from fishing and left your fish basket in the hot car for TOO LONG before cleaning the fish?
Wait, this one has happened to me… You spent a fair amount of time casting into the pond without a fly on the end of your tippet?
I went fishing with my granddaughter one time and used worms. Left them in the car with the windows up for three days in over 100 degree temps. Now I know what death smells like. Similar to FishnDaves answer. Now for his second answer. Just got back from a four day fishing trip to PA and was fishing for quite awhile. I had fish rising at what I thought was my fly. After awhile of rises but no bites, I brought in my line and no fly, although I cought see where the flie used to be by the curly area where the knot was. What an idiot I was in both instances.
On another note. Same trip. I was waiting for the guy to pick me up, we were going to ride together to PA and when he showed up, I loaded my stuff into his vehicle and it was then I realized,…I forgot my rods. So, I am going to say…Forgot your rods.
Went home and left your fish basket at the fishing site?
Ed
You decided to make a cast towards a large snapping turtle (or muskrat or beaver or duck)…and hooked it?
forgot your red worms and crickets!
Left your dentures laying on the kitchen counter 
Did you forget to check the weather forecast before you left the house?
You forgot to close a gate behind you and the farmer’s cows got out?
Bent over to net the fish, and your cell phone dived into the water?
No body has it yet.
Rick
Got to the spot with either no rod, no reel, or no flies
You forgot to ask your wife for PERMISSION to go a-fishin’? 
You went to fish a pond that no longer had any water in it?
I don’t even know what dumb move #1 was 
.
I don’t remember what my No. 1 dumb move was or my No. 2. Maybe he resets his counter more often than I do.
You locked your keys in the car with the rod still in it.
Did not check for wind knots and lost your dream fish.
Tried to lift your dream fish by the tippet and it broke off.
Snapped a tip off in the car door, or with the trunk lid.