My first job out of college was teaching COBOL. I understand the IRS is looking for instructors...
What they need is both people who understand the old systems and people who understand the “old way of doing things”.
A private sector example might be an old manufacturing company that had two computer systems, one for industrial products and another for consumer products.
While the systems may not have been expected to talk to each other, some sharp young geek in the old days may have figured out that if they did talk to each other it could save him a lot of work (duplicate programming).
So—the geek set up subroutines where periodically the two systems shared some data and used it for different purposes. Since he was getting paid for programming both systems he would not want to tell the bosses what he did since he was reading books or sleeping during all the time he saved.
Fast forward sixty years later (!) and the geek is long retired and it is time to overhaul the ancient systems.
The new programmers will probably not catch the communications subroutines (since there were not supposed to be any according to the documentation) so they will fail to duplicate it—as a result some subroutines will not run on the new system.
There will be no obvious errors at first—sometimes this stuff can take months or years to detect.