Running shoes - being used to flat paved footpaths/roads I found the slightly more uneven surfaces of the countryside walks combined with soft trainer soles made my ankles ache a bit, so I invested in a pair of walking boots ... problem solved.
Not taking account of how much energy is needed to do long, all-day walks. Took a basic pack-up of jam sarnies, banana, salted crisps and a flapjack but it wasn't enough to sustain me for a full days walking and by late afternoon I was flagging and having to eat a kindly donated Mars bar and some broken bits of Kendal Mint Cake to pick me up and see me to the finish.
Now I take a selection of things to nibble on as we walk to keep the tank topped up (it's no good waiting until you're shivering and shaking, by them your blood sugar is already too low and it takes longer to get yourself back up to speed).
I have issues with my ear. One day out walking on a beautiful sunny day, I didn't take any head cover as we climbed higher the wind picked up and was really strong and cold, I ended up with really bad ear ache, lesson learned, I now take a buff with me.
I was about to say I haven't really made any due to my idea of walking being fairly modest. Then I remembered a 10 mile walk around Grizedale forest with my brother and our two dogs. We made sure we had food and water for the dogs and completely forgot about ourselves. We did have a small bottle each in our bags but 300ml doesn't go far sometimes.
More memorable are the other people we've seen with inadequate or simply completely wrong gear for what they are doing - one in particular being the lady headed up Coniston Old Man in moccasins.
My worst mistake was getting caught in torrential rain and a gale walking between Horton and Hawes on the Pennine Way with a very lightweight goretex running jacket and no other waterproofs. It was the middle of June and I could have been forgiven because the weather until then had been dry (for the previous few weeks). I got a nasty dose of hyperthermia......not good.