Slip your mind
Definition 1 of 1
Pro Tip 1/3Accidental Forgetting
Use 'slip your mind' when you unintentionally forget something, not when you choose to ignore it. 
I didn't ignore your message; it just slipped my mind.
Pro Tip 2/3Momentary Lapse
'Slip your mind' often implies a temporary forgetfulness, as if you remembered it before or will remember later. 
I knew her birthday last week, but it slipped my mind today.
Pro Tip 3/3Casual Tone
The phrase has a casual, informal tone, implying a light-hearted or excusable kind of forgetfulness. 
Oops, it slipped my mind to bring the snacks.