Good thing I'm okay at puzzles

My roommate locked himself out of our apartment, but noticed it before he left his building. After waiting around for his building manager, he decided to come down to my office building and pick up my building and apartment keys.
He’s got a weird sleep schedule, though, so chances are he’ll be asleep when I get home. I suggested that once he gets into the building and apartment with my keys, he picks up his key, puts my two keys inside the mailbox with his mailbox key, and I open up the mailbox with my mailbox key to retrieve my two keys.
As he was leaving I realized I wouldn’t be able to even enter my building to access the mailbox if he had my building key, but then I remembered that my cell phone is also a key – I just use the numpad at the building door to dial my phone and unlock the door.
I took the elevator back to my office floor, and tried opening the front door to my office – locked. As a contractor, I don’t have a keycard. Fortunately, a round of loud knocking alerted someone and they let me in.
My head hurts.