Mistake

A brief discussion of the following koan:

"A monk once asked Unmon, 'The radiance serenely illuminates the whole vast universe...'
Before he could finish the first line,
Unmon suddenly interrupted, 'Aren't those the words of Chosetsu Shusai?'
The monk replied, 'Yes, they are.'
Unmon said, 'You have slipped up in the words.'
Afterwards, Zen Master Shishin brough the matter up and said,
'Tell me, at what point did he slip?"