I have a 1951 $50 FRN that has that part on it “...and may be redeemed in lawful money...”
A buddy of mine does alot of numismatic stuff so goes to the bank regular to buy rolls of halves, etc.
The teller told him some kid came in and wanted to cash a coin - so they offered him face value for it, and he took it.
The bank was out one $50 FRN but the kid was out something like a 1922 Gold Eagle that he found in his grandads dresser drawer.
And this was fairly recent.
Kinda outrageous, imho.
Banks are kinda outrageous all the time. (read Wells Fargo's press release about mortgage defaults this morning)
Another friend of mine worked security for a bingo parlor. Every week there would be a few United States Notes or Silver Certificates spent, usually obvious by the different color seals, and many had a rust stain from a paper clip on them. They dated back as far as pre WWII.
He was pretty sure they were being spent by an elderly lady there, one who remembered the bank collapses of the Depression and who had a 'stash' of money.
If the stuff hits the fan, economically, I figure Gold and Silver will retain their purchasing power to some extent. Maybe clad coinage will 'trade' at a discount, if not they can always be drilled out and used for washers or something (I know, bimetallic washers would likely promote cathodic corrosion, but this is a fairly dry climate).