If there is one thing that should scare investors out of a crowded trade, its a warning from veteran Wall Street-er Art Cashin.

As a reminder, speculative investors have never been more one-way positioned short in Treasury yields...

And UBS' Art Cashin warned during a CNBC interview yesterday that it could be a bad day for the markets once the yield on the benchmark 10-year Treasury hits 3 percent:

"That 3 percent level is both a target and a kind of resistance. Everybody knows it's like touching the third rail," 

"The assumption is once they do it, all hell will break loose. So we'll wait and see."

The sharp moves seen Wednesday were probably due to "our friends, the long-lost 'bond vigilantes,'" Cashin told "Squawk on the Street."

"We're going to need a couple weeks to see if the bond vigilantes really are back or not," Cashin said.

"Or whether it was simply a fluke. But remembering what bond vigilantes look like, it certainly had fingerprints on them."

But for now, since Cashin's warning, it seems more than a few investors have taken some short bond chips off the table as Treasury yields have tumbled...

As bond yields have finally caught up to the inflationary signals from copper/gold...