So I'm safety piloting for a friend today who needs some approach currency. She brought an old (70-something) V-tail Bonanza. On the fuel tanks, the caution area is a good 1/3 tank or so with the annotation "do not take off...etc" next to it.

What's that limitation for? 1/3 tanks seems like quite a bit if you just wanted to go do a local hop, say touch and gos or hop over to another airport to grab lunch. FWIW, the aircraft has 80 gallon tanks I believe, so we'd still be looking at 27 or so gallons. I can't imagine there's anything in the tank design that would be dangerous about taking off with 20+ gallons of fuel onboard.

And no, it's not in the yellow, it's damn near full of fuel.