As many rolls of coins as I've searched it is extremely rare to find proof coin in circulation. I picked up one of my kennedy half books the other day to see what I was missing. When I opened it I noticed that 1976D looked proof like. The reason it looked like that because it was a 1976S. It been in there about a year and placed there by me when I was roll searching halves. Just goes to show that your eyes start too go when you search to many rolls. The same day when searching nickel rolls a coin came out of one roll and I thought it was a BU 1942-45 nickel but it turned out to be 1971S proof. Since things come in threes I waiting for the next proof coin.
I've found two proofs (both quarters) in circulation. Both dated in the early '90s, but I found them in the late '90s.
I found a 1982-s proof nickel in the mid 90's. The amazing thing is that it was still very proof like. I t had probably just been liberated from some collection by some kid wanting to buy candy.:smile
I've found dozens of proofs in circulation. The most amazing one was back in about 1996 I found a '68-S quarter worn down to VG. The proof singles got down as low as about 120% of face value back in 1995 and the entire sets got close to 150% of face. Buyers were hard to find so motivated sellers would sometimes just bust the sets up and spend them or, more typically, save only the nice coins. Some of the older sets have been placed into circulation in fairly substantial numbers. Most of the proofs going into circulation now days are culls from recently broken sets. When the total value of the coins in the set rises above the value of the set then they'll be destroyed in large numbers. Buyers have no interest in coins with tarnish or marking so these often just go into pocket change.