Cast Undercover Operative by paying ({2} magic symbol) ({U} magic symbol) ({U} magic symbol) , choosing to have it enter the battlefield as a copy of Enduring Scalelord.
Undercover Operative enters the battlefield, triggering Cathars' Crusade, putting a +1/+1 counter on each creature you control.
Enduring Scalelord and Undercover Operative triggers.
Resolve the Undercover Operative trigger, putting a +1/+1 counter on itself.
Enduring Scalelord triggers, putting a +1/+1 counter on itself.
Repeat from step 4.
Results
Infinite +1/+1 counters on Undercover Operative and Enduring Scalelord.