1. The best lunch spots will be found either at 11am and/ or 3pm and certainly not at lunchtime.
3. Rocks that feel dry to the hand will feel wet to the bum.
5. Taking full waterproofs is the best guarantee of good weather.
6. The route you want will always be over the most awkward fold in the map. Or, in the case of the Seathwaite route up Scafell Pike, it will be on 2 different maps.
7. When you are utterly knackered and hauling yourself up "the hardest climb you've ever done" a fell runner will breeze past with a cheery "Hello".
8. You will need a pee in the most exposed and barren part of the hike.
9. When faced with a difficult spot of navigation in the mist you will inevitably have an audience. They will watch whilst you have a hissed exchange of opinions and they will continue to watch as you disappear off in one direction only to reappear and walk past them in the opposite direction 5 minutes later.
10. However well equipped you are there will be someone in the car park with bigger/ shinier/ newer.
11. You will have a good phone signal until it's misty, dark or both and you want to quickly double check your location. At this point all satellites in the northern hemisphere will hide themselves from your phone.
12. However remote or obscure your route, someone else will decide to hike it at exactly the same time as you and will usually be chatting loudly.
13. At the top of the most popular routes a large number of people will feel the urge to phone home to say "You'll never guess where I'm calling you from. I SAID YOU'LL NEVER GUESS WHERE I'M CALLING YOU FROM."
14. Removal of layers of clothing is directly linked to changes in the weather - the second you remove a layer, the temperature will drop requiring you to replace it again, at which point the sun will come out.
Well there's my 20 to kick off with, though I feel sure there are many more - please feel free to leave comments and tell me what I've missed.