Funny.The most well-known example given is the Our Father:
Matthew:
“Our Father in heaven,
hallowed be your name.
Your kingdom come,
your will be done,
on earth as it is in heaven.
Give us this day our daily bread,
and forgive us our debts,
as we also have forgiven our debtors.
And lead us not into temptation,
but deliver us from evil.”
Luke:
“Father,
hallowed be your name.
Your kingdom come.
Give us each day our daily bread,
and forgive us our sins,
for we ourselves forgive everyone who is indebted to us.
And lead us not into temptation.”
Luke’s more terse version of the prayer is believed to be more original than Matthew’s, because it is thought unlikely that Luke would have pared down the Matthean version so familiar to us. The Q version of the prayer, then, will have probably looked more like the Lukan version, and the extra Matthean parts will be additions distinct to Matthew, such as “our Father in heaven” (since the phrase “your Father who is in heaven” is apparently a favorite of Matthew’s). Likewise the Beatitudes: Luke’s “Blessed are you poor” is thought to be more original than Matthew’s more spiritual “Blessed are the poor in spirit.” The reverse idea, wherein Luke derives his more down-to-earth version from Matthew, is thought to be unlikely. In these and other cases, it is felt that the Lukan version is less characteristically Lukan than the Matthean version is characteristically Matthean, a situation easily explicable if both are independently redacting an unknown source, but implausible if Luke is redacting Matthew.
I always attributed the difference to two people in the crowd not hearing every word.