What's the Ruling of Forgetting to say Bismillah upon slaughtering?

Yes. If you left out saying Bismillah out of forgetfulness, then eat it. Because Allah excused this [Muslim] nation for their mistakes and forgetful actions. [As Allah said]: "Our Lord! Punish us not if we forget or fall into error." [al-Baqarah 2:286]. If you forget the intention, then it is in your heart, make the intention. But if you forgot it at the time of slaughter, then you will not be blamed and the slaughter is permissible. However, if you were to leave it [i.e. Bismillah] intentionally, then this is the one who's slaughter is not accepted. [As Allah said]: "And do not eat of that upon which the name of Allah has not been mentioned."" [al-An'am 6:121]