A program 'bug' leads to a series of weirdness!

Some narrative here:

• WHITE Knight moved to F5 to put King in Check
• (Here's the bug): BLACK Pawn takes WHITE Knight exposing King to Check from the WHITE Rook
• Now what to do? We are in chess Bizarro-world!
• I show this, incredulously, to my wife (who knows a thing or two about chess!)
• So I capture the BLACK King! (A piece that can be mated but not captured!)
• I keep playing just to see what will happen (below)

Somehow I mate the BLACK King ... and it's not even on the board!

Giving up for the evening - I can't wrap my brain around this!