Pens and stuff like that. I also donated stuff to work. Sometimes I covered the cost by padding my time sheet, but not always. I did a lot of stuff that wasn't anywhere close to what I was hired to do, and not always within my comfortable skill set, so I billed what the job should cost if I knew what I was doing. IOW, I might have spent 5 hours on a project, but I'd put 3 hours on the time sheet. Materials I brought in were sometimes converted to time, and weighed against market value, and effort I put in.
tl;dr
I don't owe anybody anything. Sometimes I made out, sometimes the company made out.