Being amused at just how much prep something like Bloody Omaha or Dead's Army takes to be done properly. And why you playtest...
Making sure you have all the figures you need? Sure. Riiight. And who thought his German mortar sections were painted? That would be me :D
Making sure you have all the dice you need? Have you any idea how many shock/kill dice Bloody Omaha needs - no? Neither had I until Sunday!
Tokens? Uhuh. See above.
Unit markers? Well, yes. Till we figured out what chaos a beach with elements of TWELVE assault sections on it is, even IF they're based in 4s.
Player handouts? QRS? I'd have done those anyway, honest :D
Remembering all the bits of rules that are likely to be needed, that are in obscure dark corners of a rulebook with no index (sorry Rich!)? Playtest.
Minor scenario redesigns? Did I mention playtest?