There is no such thing as “casually” forget to mention a sin. If someone is so casual, it is very likely that they do not see Confession as the wonderful sacrament that it is.
It is very possible that a person really forgets a serious sin - but this would most likely be if it had been a really long time since their last confession. If we commit a mortal sin, I’m pretty sure, since in order to commit one the person has to know that it is a mortal sin and choose deliberately to do it anyway, that we would not forget about this, unless a lot of time has passed. So, to “casually” forget to bring that sin to confession really means that they, for whatever reason, have no real intention to confess it.
There is no such thing as an “imperfect confession”. It is either a good confession or it is a bad, sacrilegious confession, meaning that, even though the priest said the words of absolution, there is no absolution, and there is another mortal sin on the person’s soul.
Even if you have a doubt about whether you really forgot or deliberately withheld the sin, go to confession and confess - not just the sin you did not confess at the last confession - but all mortal sins since the previous good confession and that your last confession may have been a bad confession because of the sin you did not confess.
You will feel so much better, knowing that you have put things right with God.