One of my worst mistakes was at a tournament at Lake Martin one October. I really didn't forget my rainsuit and rubber boots. They were in the van. But the weather guessers had predicted the rain would not start until about 5:00 PM, well after our 2:00 PM weigh-in. So I left the rain gear in my van at the ramp.
Sure enough, at about 9:00 AM it started sprinkling then a steady rain fell. I was soaked within 30 minutes and shivering in the 55 degree air. I toughed it out for about 15 minutes then ran the five miles back to the ramp for the rain gear and dry clothes. I was able to fish comfortably the rest of the day.
Fishing items are usually not forgotten but some other things can easily be left behind.
First Aid Kit
A first aid kit can be critical if needed. Keep one handy. It should contain antibiotic cream, band aides and bandages, antiseptic and aspirin. You should also have a few pills of your prescription medicine just in case. Some nose spray is good, and if you are allergic to insect stings an epi-pen is critical. Make sure all the items are fresh and within expiration dates.
I have forgotten a cap more times than I care to remember so I always have some extras in the boat. A cap with a visor really helps on sunny days, and I also carry a wide brim hat that protects my ears and neck from the sun. My caps have to be solid on top since my thinning hair no longer will protect my scalp from the sun if I wear a visor or ventilated cap.
No matter how warm it feels in the truck, it is going to be cooler on the water. After several hours of fishing even what started as a comfortable day can turn miserable. Carry a light jacket with you just in case. Your rainsuit jacket can help in a pinch but they are usually not insulated in any way so they are not real warm. They will break the wind and help, though.
After my experience at Lake Martin I always have some kind of rainsuit in the boat. You can get a cheap one and keep it in the boat, just be sure to check it often and replace it when needed. A rainsuit that leaks doesn't do any good. I like bibs and a long jacket to keep my waist area dry. A jacket and pants often ride down and let water in.
If your feet are wet the rest of you can not be comfortable. I got a pair of cheap rubber overshoes at a feed and seed store. They go on over my tennis shoes or can be worm without shoes in the boat in a pinch. They have saved me from uncomfortable feet many times. I have even slipped them on over a spare pair of socks I keep in the boat after stepping in the water or having a wave wash over the side or front of the boat and wetting my feet.
Bugs can bug you so carry some kind of repellant with you. A spray can is convenient but they can clog and not work. A foil pack of cream type bug juice is always fresh and ready to use. Even if you are fishing where bugs are not a bad problem, have some just in case. If you are fishing areas you can not fish without some good bug repellant.
It is way too easy to forget sunglasses when you get in the boat before daylight and head out just as it gets light. A spare pair of sunglasses, even fairly cheap ones, are very welcome when the sun gets bright. Even better if you have an older pair of your favorite glasses you can keep in the boat.
I used to be able to thread a hook eye on the line with no problem but my older eyes now need help. I have prescription bifocals but keep a pair of cheap drug store reading glasses in the boat just in case. If my glasses are forgotten or lost overboard I have a back-up when doing close work.
Try to remember everything you need with a checklist, but have a back-up plan if you forget something you need or want.