God knows how this happened, considering overflow bugs have been known about since the days of Pac-Man, and have been bugs in games such as Wiz ‘n’ Liz, Golden Axe 2, and several other games where a number going below zero verts it straight to 255, 65,535, or 4,294,967,295. XBox players who tried to get the Battlefield beta on the market saw the staggeringly high number.
Let me explain this from a more technical standpoint. 255, 65,535, and 4,294,967,295 are the highest numbers possible in certain bit numbers, with 4,294,967,295 being the highest possible number in 32-bit. In some programs not properly coded, an error can cause a number that goes below zero to wrap around to the highest possible number. Multiple games have a bug or two related to going over or under the limit that can be beneficial or ruin your entire game altogether. A few examples can be found on this page: http://tvtropes.org/pmwiki/pmwiki.php/Main/Cap
Regardless, this is simply a display error and users were able to get into the beta for free.