Can A Forgiven Sin Be Un-forgiven?

Can a forgiven sin be un-forgiven?

Reference Matthew 18:23-35

Jesus speaks in parables often throughout the Gospels.  He does this to educate people, giving them real world scenarios of what God expects of us.  Matthew 18:23-35 tells the parable of the unforgiving servant.  In summary:

* The kingdom of heaven may be compared to a king who wished to settle accounts - v23
*A slave did not have the means to repay the debt, so the king commanded him to be sold - v25
*After begging for mercy, the king had compassion for the slave and forgave him his debt - v27
* Someone else owes a debt to the slave and begs the slave for mercy - v29
*The slave refuses mercy and has the debtor thrown in prison - v30
*The king finds out about the slaves actions - v32
*Angry, the king hands the slave to the torturers until is debt is paid - v34
*Jesus says the heavenly Father will do the same to us - v35

In the parable, this man's sin was forgiven in verse 27, but then he was held accountable again in verse 34.  According to Jesus in verse 35, we are held to the same accountability.  If we have been forgiven a sin, then later in life we do not forgive others for sinning against us, we will be held accountable for our sin.

Thoughts?