You slightly misremembered the invisible gorilla test, a classical study about inattentional blindness.

https://en.m.wikipedia.org/wiki/Inattentional_blindness#Invi...

Somewhat fittingly "the basic Simons and Chabris study was reused on British television as a public safety advert designed to point out the potential dangers to cyclists caused by inattentional blindness in motorists."