...A 143 tear old law could cause more trouble than anyone thought possible. A mandate on the books since 1870 called the "Antideficiency Act" could get a government official who does something as innocent as checking work e-mail "thrown in jail or fined or fired OR all 3".

This MSNBC story "143-year-old law stirs fears during shutdown" explains it a little better.

"In shutting down the government, most memos cite the law as the reason. The Government Accountability Office says employees who violate the Antideficiency Act may be subject to disciplinary action, suspension and even "fines, imprisonment, or both."
CNBC has learned that in several executive branch departments, high-level staff members review individual decisions about what government activities to allow for fear of running afoul of the Antideficiency Act. One White House official said he has advised his employees not to check their email or cellphones. Under the act, even volunteering for government service is expressly prohibited.

In a memo to his department employees today, Treasury Secretary Jack Lew cited the law as the reason for reduced staffing.
"For the duration of this impasse, as required by the Antideficiency Act and directed by OMB, the Department will be required to operate with only the minimal staffing level necessary to execute only certain legally exempted activities," Lew wrote."

I can think of a few people who should check their email often...say 535 of them...;)