I've had a lot of them, these are not from 1 or 2 trips.. this is a lifetime of things I didn't plan for and costed money....
0: didn't know gratuity wasn't included in meal plan and wow that's a lot of money for tips on top of the meal plan cost..
1: completely underestimated how dehydrated we might be when back in our room for the night and we were tired of just water and got sports drinks to help hydrate faster. We were cramping and not doing well.
2: forgot to pack stuff to handle blisters and needed to buy things.
3: a very rainy trip we needed to do laundry 2 extra times (soap + laundry expense)
4: same rainy trip, a pair of sneakers so wet that even after dried smelled horrible and needed to be replaced.
5: ponchos we brought from a dollar store didn't survive and we had to get more while there.
6: we enjoyed
MNSSHP so much we baught a second ticket.
7: daughter dove out of the way twisting her ankle instead of being run over by kids pushing a wheelchair and needed a wheelchair or
ecv the next couple days to help stay off her feet and recover without sitting around doing nothing.
8: finally got tickets to kttk tour when someone canceled.
9: decided to take a taxi to an early morning tour instead of disney transportation.
10: starbucks and joffrey's coffee in the park.
11: ran into something perfect for a holiday present for a family member.
12: decided to check out universal one day.
13: decided to see la nouba to have somewhere without rain on a very raining trip.
14: went a little crazy trying out snacks we've never had
15: starting a popcorn container collection (and there are a lot of them, and they may or may not be different every trip).
16: got tired of the similar food we kept finding and splurged for a nicer meal just to have something different to eat 1 meal
17: there will be something for sale that will call out your name and you'll never use again. (my daughters is some limited release mickey ears)
18: something you had no clue was at disney and someone else you randomly talked to recommended you do it.
19: something special that you consider do not miss started after you thought you were done planning and found out about while there.
20: disney changed prices on you mid trip (parking, cost of food, drinks, merch, etc).
21: we really enjoyed disney quest
22: penny machines.
23: massage chairs (my favorite ride)
24: spent and saved money at the same time by using garden grocers after we arrived so we ordered thigns for breakfast and bottle beverages.
25: my daughter suddenly got sick as we left the park and was not able to travel as we were leaving the park to head to the airport. I had to scramble for a room, change travel plans, medical supplies, visiting a doctor, transportation that was more direct and faster, food, etc for those days. Some of it I was able to get back on
travel insurance because I had the paperwork from a doctor that she shouldn't travel..
26: stuck at disney because of weather and canceled flights (I know, it's horrible being in florida extra time while back home it's a blizzard

).
27: broken phone charger cables needed to be replaced.
28: loved H2O and got better quality versions while at a value

29: didn't expect the weather to be good enough for pool and needed to get clothing for that.
30: wasn't expecting the night to be so cold and decided to buy sweatshirts instead of calling it early (was a party we paid for, wasn't going to miss it because we planned bad).
31: the snow storm of 2010, wow we needed stuff.
32: stuff we just bought in the moment we had no use for.
33: completely missed an ADR and got the fees.
34: 6+ people eating at a table service is an automatic 18% gratuity no matter how horrible they served you.
35: hadn't traveled in awhile and forgot about tipping maid service, bell hops, etc..
36: the arcade was really fun...

37: the games in the dino section of AK.
38: buying stuff at the resort because I was too tired to get my drink cup from the room and come back to fill it up instead.
39: buying stuff at the resort because we didnt' have time to get our drink cups before they closed..
40: really thirsty and using the vending machines at bus stop or resort.
41: finding out the flower and garden preview buffet style lunch event was happening while we were there and getting tickets for it.
42: needed another bag and baggage fees to get things home.. (we packed tight and forgot about room for anything we got home)
43: Giving my daughter some freedom and some extra spending money to do a park on her own.
You didn't ask this but things i've learned about to help offset costs that I think people overspend on prior to arriving:
1: basic medication is available free from first aid. (headache, ibuprofin, allergy, band aids). Basically anything needed to keep you from leaving and not spending more money was free.
2: a cup of ice water is free from all counter services.
3: some candy shops, especially in springs has free samples to try.
4: on longer trips we like to have several unplanned days. We don't purchase park tickets for those days (or book fastpasses). if we decide we want a park, we add the tickets on for whoever is going into the park. Saves money overall. We are ok without fastpasses but take what we can find.. (it's really easy to get some btw).
5: don't purchase park hoppers or water parks and more until you actually need it while at WDW. It's the same price to add it on while there, not everyone your travel with needs to have it and if you decide it's not worth the cost you just saved a lot of money. You can't get a refund for unused park hoppers or water parks.
6: finding out disney's over priced cigarettes are actually cheaper than nyc prices.
7: goes with #33 above: call them up the next day and they will waive the no show fees in most cases.
8: give some magic. if you have extra meal plan credits you can't use, near the end of your trip surprise a family and ask them to join you for a table service. You overhear families talking and you'll find a family that is spending their life savings just to be there and no possible way they can do more than bring their own food.
We didn't experience this one but we met an amazing family that did.. They got food poisoning and severely dehydrated. Spent 24 hours in the hospital. It was week 2 of a 5 week trip and they were from canada. They couldn't afford the health care and had to go home early for treatment. If they had travel insurance they would have had coverage for illness and stayed...
AND THE ULTIMATE EXTRA UNPLANNED COST:
We enjoyed our fall trip so much, that we came back for 9 days 2 months later for xmas....