Recently (since the Year 2000) in the United States of America, Federal and State agencies have allowed the re-hiring of retired employees without the loss of their retirement benefits. Such a "re-hire" is referred to as an annuitant. Often a maximum number of hours per year which the annuitant may work is specified.
For instance, the State of California has posted its guidelines:
This concept often allows agencies to benefit from the experience of retired employees who may be relied upon to share their experience and training with new hires, or to supplement or "bridge" areas where personpower is needed but not currently affordable.