I noticed a problem similar..well, related perhaps... to this a while back when I was whipping up a tile engine. In debug mode the game would crash when walking off the edge of the map, and in release it was wrapping around to the other side of the array (only when exiting the map vertically though, strangely enough)
Of course, that was an older version of Blitzmax.
Of course, that was an older version of Blitzmax.