I learned a long time ago, there will be plenty of cheap sweatshirts if it gets cold - case in point: New Light tour to the West in 1986. We had 3 free days during each tour and the day I remember needing a sweatshirt was July in San Fransisco. I've never forgot that lesson and always pack extra wind gear for that city.
Also, there will always be something to eat - case in point: the peanut butter on my 1994 trip to Istanbul was totally unnecessary. Once I tried the fresh bread and then found that the wine went well with the bread, there was very little peanut butter consumed that summer.
If I run out of skiveys, I will have dry Tide soap to wash out my things in the bathroom of whatever hostel I end up in for a two night stay - case in point: three nights on the 5th floor of a hotel overlooking the Black Sea in 1997. Plenty of clothes hanging all over the room, but lots of wind to keep me cool and dry the clothes.
So, while packing, I keep in mind that the Dramamine for G#3 is quite necessary and will not be shed during repack, but the box of ring pops may not have to be so lustful. Every piece of clothing I'm taking can be worn with one pair of blue Birkenstock that will allow me to get on the plane standby, maybe even in business class. AND, the walkie talkies are going with us in place of any more of my shoes so that I can plant one on each kid and one for myself in case they get separated from us during any of the shuffles.