What is the best thing you have ever passed up not knowing?  

We all have passed up things
Yea I passed up a couple of Eames Bikini chairs with
original dowel legs and a couple of Bertoia diamond chairs.
This is when I first started collecting. My new motto is
is it is cheap enough go ahead and buy.

I considered buying this to use as a plant stand
but didn't, because it cost $30 or so, and I didn't really need it.
Ah, youth.

Wood Lamp
A nice oval wood laminate lamp last fall at a resale shop. Kept passing on it, eventually it was half off...$1..hemmed and hawed one more day and finally decided to just buy it. Went back and it was gone.
Two weeks later at a reputable dealer, and my first visit...there it was. Priced significantly higher...add three zeros. Listed as unattributed. Imagine how I felt.

and how about those things...
and how about those things that you bought and later regretted...

regret
Don't even ask! I alone could probably turn your stomaches on junk I purchased.

P40 tecno
there's one i'll allways regret: a P40 chair by Osvaldo Borsani for Tecno. It stood in a junkshop for about 60?, but the foam was almost fossilized and i didn't know the design
back then.
tragedy is: i really come to like that specific chair..

Gotta take a risk
If you haven't ended up with some regretable purchases
them you are not taking enough risks.

I blocked the memory of most...
...of my missed opportunities. Too painful to remember. But one that I do recall was a pottery vase that I thought was ugly at first glance but that grew on me. It was at an auction. I meant to go look at it more closely but decided to pass since I'd bought a really great rosewood table and six Kai Christensen rosewood chairs at the same sale and was tapped out money-wise. I took some of the chairs home and came back for the rest and the vase had been sold. Some guy had it sitting next to him. That's when I knew that I really wanted it---when I couldn't have it. Wah.
Another one was an elegant little loveseat by Dunbar. It was Wormley-esque. Also at auction. The more I looked at it, the more I liked it, but I wasn't sure where I'd put it and I knew I didn't NEED it, and I wasn't sure what it was. So I passed on it, but then I kept thinking about it. I actually tried to track down the guy who someone said they thought bought it but that came to nothing. Turned out it was someone else anyway, and it got sold in another auction probably within a week. Oh well.