There is a little thing known as LWOP or LEAVE WITHOUT PAY. That may not sound like much, but at the end of your Career, it can have a serious effect on your Severance Pay. LWOP are counted towards your Pensionable time, but when your Severance Pay is calculated, it is only for Paid Time. If you have any Leave Without Pay, those days are deducted from your total days of your Career. Therefore, at the end of twenty, twenty-five, thirty, or whatever years you will be short those days to make a complete year. That means you loose a years worth of Severance Pay, which could be a couple thousand dollars out of your pocket.
There are ways around that. One way is not to have them before you join, or to have Leave that will take you past your Release date.