Garage Sale - S7-E18
Corrected entry: As Michael is getting ready to propose to Holly, he stops in the foyer and says, "This is where Toby announced that he was going to Costa Rica. Happiest day of my life." This is a discrepancy, as Michael was in New York partying with Ryan and Dwight when Toby made this announcement.
Correction: Michael wasn't there when he announced it to the office so he may have heard it in the foyer from Toby there after for the first time.