Due to the intricacies of the federal government employee time keeping and sick leave processes, it's possible to have left over sick days that are either used prior to or forfeited upon retirement. In my case I had 9 days to use or lose from August through the end of the calendar year at which point I was retiring.
I started out with fairly benign reasons for calling in sick, but as time went along I got more creative. For a while I claimed I was suffering from various maladies/illnesses I nicked from episodes of "House," including Addison's Disease, Wilson's Disease, psittacosis, brucellosis, etc. No one ever questioned these as it was common practice to use those sick days playing out the string rather than lose them.
The week between Christmas and New Year's, I still had one day left. I called in on the morning I decided to take off my last day of sick leave and solemnly told the young female admin type I wouldn't be coming in because I was lactating. I'm still unsure if she didn't know what that meant, didn't understand me, or just didn't care. All she said was, "Thank you Mr Duke, I hope you feel better. "
Two days later I retired.