Venial sin misjudged as mortal sin

[Irenaeus0628]

If one commits a sin deliberately, even if he thinks it is mortal, but actually the sin was not a grave matter, then did he commit the sin mortally or venially?
I know that if one commits a sin deliberately while being doubtful whether it is mortal or not and it was actually mortal, then he is guilty of mortal sin. But what if that was actually venial?

 

[Stephen_says]

A venial sin can become a mortal sin depending on the disposition of the agent. From the the Summa:

Nevertheless, since moral acts derive their character of goodness and malice, not only from their objects, but also from some disposition of the agent, as stated above (Question 18, Articles 4 and 6), it happens sometimes that a sin which is venial generically by reason of its object, becomes mortal on the part of the agent, either because he fixes his last end therein, or because he directs it to something that is a mortal sin in its own genus; for example, if a man direct an idle word to the commission of adultery. In like manner it may happen, on the part of the agent, that a sin generically mortal because venial, by reason of the act being imperfect, i.e. not deliberated by reason, which is the proper principle of an evil act, as we have said above in reference to sudden movements of unbelief.

http://www.newadvent.org/summa/2088.htm#article4