I can't say what happens here in the states if you actually land on a military runway by mistake, but I do have some idea (secondhand information--one person I knew actually did this, and in a second case my wife was working at the facility at the time), of what happens if you mistakenly taxi past the "red line" onto the military ramp at a civilian airport, that is also home to a military installation.
People, with very little senses of humor, but very big M-16s, greet you before you can even step out of your airplane. What they lack in humor, they do make up for in curiousity, evidenced by the fact that they ask you lots and lots of questions...