Matthew 7.6 says "Do not give what is holy to dogs - they will only turn
and attack you.  Do not throw your pearls in front of pigs - they will
only trample them underfoot."

This verse is in the context of calling people to not judge others.
Therefore I think it is likely that it is a copy mistake.

My guess is that the orginal should have read:

"You have heard that it was said 'Do not give what is holy to dogs - they
will only turn and attack you.  Do not throw your pearls in front of pigs
- they will only trample them underfoot.'

But now I tell you:  Honor all people and withhold the wisdom of God from
no-one.  For what is truely holy turns all who hear it to see the kingdom
of heaven."