This story comes from Caitanya Simha Prabhu ACBSP (Venezuela)-
There was a disciple of SP in Caracass temple, Venezuela, who was called Pitavas Prabhu (I may have remembered the name wrongly) and who regularly got on everyone's nerve. He joked on other devotees and played pranks on them, and no amount of neither correction nor chastisement from neither devotees not authorities helped. He was impossible.
Then Srila Prabhupada visited Venezuela in 1975 and stayed in temple for a short while. Devotees were in ecstasy. At that time both Caitanya Simha Prabhu and Pitavas Prabhu served in the kitchen. One nice day as they were cooking the meal Srila Prabhupada entered the kitchen. That was it -the perfection of their devotional lives - Srila Prabhupada came to bless their service! They all fell on the ground like sticks paying their heartfull obeisances to their beloved spiritual master and then got up staring at Srila Prabhupada.
Srila Prabhupada had no particular expression on his face. His righthand was in his japa bag. Then he pulled his hand out of his japa bag and took a short walk around the kitchen pulling his forefinger over everysingle surface he could reach - tables, shelves, window shelves, above the door... and by the time he finished it the upper part of his forefinger was black from the dirt accumulated on those surfaces. It all lasted no more that several seconds.
Srila Prabhupada turned around, walked up to Pitavas Prabhu, wiped his transcendental forefinger on Pitavasa's cheek leaving a thick black line,and walked out of the kitchen. From that day on Pitavas became so grave, sober, deep, andintrospective, so full of respect to the devotees, and so attentive in his execution of devotional service that he never again neither made another joke nor played another prank on any devotee.