"It's hard to play poker with someone you haven't seen before," said Man Group chief executive Luke Ellis last week, referring to the crazed price moves driven by amateur day traders. The pros must be wondering: is it sometimes smart to do dumb things?

It has certainly been profitable lately. If you were quick, you could have made a bundle by betting on bankrupt stocks such as Hertz, JC Penney and Chesapeake Energy. Indeed, if you very quick, you could have bagged a 13-fold return if you had guessed that the day traders would pile into Chinese company FANGDD Network, which they seemingly confused with the FAANGs (the widely-used acronym for Facebook, Amazon, Apple, Netflix and Google).

Yes, this is crazy, but betting against it may be just as risky. The old line about markets staying irrational longer than you can stay solvent is particularly timely right now.