Young Teacher, The Subject (гет)

When a fluke of mind magic and the whims of chaos led Severus Snape to see something he was never meant to see -- his student, his goddaughter, his Mary Elizabeth, coming apart in the arms of Blaise Zabini, calling him by Severus's own name -- he swore to himself he would never think of it again.
Unfortunately, that's turning out to be easier said than done.
Or:
Severus Snape inadvertently saw a memory of Blaise Zabini fingering fem!Harry Potter and can’t get it out of his head while he’s with Aurora Sinistra.