They both remembered his own not quite perfect double-triple hat trick spell exam when he did his N.E.W.Ts not so many years ago; she remembered how focused he had been, how thin and how pale; he remembered how his rivulet actually had been a full spray of hot evaporating liquid drops and how he had had to add an extra transfiguration spell to turn the cloud that had formed into an actual rivulet. These memories might have interfered with what happened later but when Severus watched Minerva check the quality of each and every coil of ribbon she had ordered by carefully touching their surface and tearing at them a little, he observed how the dexterity of her fingers resembled that of an expert potioneer and an unbidden image of what these fingers might do if applied to the right places of his anatomy arose in his mind. It was a moment he often told her about in a quick whisper when stroking her hand under the staff table so that none of their students or fellow teacher would notice. On that day in Gladrags, however, she did not know that he was half-aroused and half-panicked when he slowly stroked her finger as she let it glide over a smooth green silk ribbon and whispered in her ear, “You do know that they are singing ribald songs about your double-triple hat trick exam in all wizarding pubs in Britain to the effect that a couple of proper wizards wearing nothing but lace underneath their robes do not need transfiguration to turn their snakes into rods and that with a little bit of mutual help, the rivulets will follow.” This was actually true and she had heard the song many a time but to hear such filth spoken by his voice in such a public place was unexpected. She leant into him and felt his erection and thought, yes I want this.

Of course none of this ended well, neither their first affair or relationship, nor their second, despite their friendship, the way they enjoyed each other sexually and the way they complemented each other. While they grew closer over the years, and a few select friends had been made aware of them being an item of sorts, it had all come to an abrupt stop when Dumbledore died and Severus Snape became headmaster and seemed to prove to be Voldemort's lackey after all. And then that turned out to be a lie but one she had been made to believe and she found that hard to forgive. They did find each other again during the time of the restoration of Hogwarts when she was headmistress but things weren't the same. When after a little less than a decade, he had admitted to an affair with Hermione Granger-as-was-to-be-again and told her they both had decided to leave their respective partners to start afresh together she had not even been very surprised and had agreed that that was probably for the best. She was getting on in years and it was time to concentrate on what she did best, teaching and running a school without the distraction of a volatile and much younger lover. Who needed more attention than she had time to give. Who she was better off not to have in her private life. Who she never realized until it was too late was the love of her life. Whose absence in her private life caused her to feel a pain and a sense of betrayal that went deeper than the one she had felt when she thought he was Voldermort's man. Not that she told him any of this. Instead she suffered in silence, burying herself in her various tasks and adding more responsibilities to her schedule. In their professional lives they continued to complement each other and at each and every bloody staff-cum-significant-other party over the next two years, she smiled like a bloody Cheshire cat and lied more than she had lied during Voldemort's apparent reign over the school. Every night she went to bed with the hope that maybe if she was lucky she would not have to wake up the next morning. And then she retired.

Your style in that last paragraph nicely reflects the brittle bitterness and pain Minerva feels; phrases are both clipped and flowing, suggesting her simultaneous passion/anger and control, until we slam into the wall of the final sentence.

she smiled like a bloody Cheshire cat and lied more than she had lied during Voldemort's apparent reign over the school

Great line.

The exam song story made me laugh aloud.

Thank you. I'm glad the pacing and the closeness of funny and sad worked for you.