
Slip your mind
Accidental Forgetting
Use 'slip your mind' when you unintentionally forget something, not when you choose to ignore it. 
Momentary Lapse
'Slip your mind' often implies a temporary forgetfulness, as if you remembered it before or will remember later. 
Casual Tone
The phrase has a casual, informal tone, implying a light-hearted or excusable kind of forgetfulness. 