Forgetting to bring my extra pickaxes or forge materials... so I get stuck with one pickaxe worth of mining.

And as someone else said: food. I usually remember to bring extra, but even then it's often my limiting factor for excursions. I had developed "lean inventory" habits due to portals, and now try to work against them.

I'm playing nomap, noportal, permadeath... and overall it has been surprisingly liberating. No map means I don't feel compelled to expose every bit of fog-of-war. No portal means no more "oh, I forgot this... voom -- voom... oh ♥♥♥♥, and that..." -- I think ahead instead of being a stupid monkey running on impulse. Permadeath means no need to worry about spare equipment/food or even a spawn-bed (though I do this when far from home, due to possible crashes, which have screwed me when I didn't set my spawn nearby). I feel more nomadic -- build/live where needed... ultimately I have a main home, but I'm not so anchored or dependent on it. I'll get back eventually... with a longship full of resources.