Disclaimer: I do not own these characters, they belong to JK Rowling.
Also, it wasn't me who came up with the idea of Slughorn/Lockhart. I was listening to Slashcast and they started discussing this pairing, which sent the plot bunnies rabid.
If he thinks hard, Slughorn can remember Lockhart.
He can remember shy blushes, soft blond hair and bright blue eyes begging for attention. He can remember pouting lips wanting kisses and smooth skin wanting to be stroked.
If he thinks hard, he can remember a Memory Charm somewhere in the mix but he can't remember which one of them cast it. This disappoints him - mostly late at night when he is in bed - because he would love to know what happened with dear little Gilderoy.