As I knew would happen eventually, I've run into the well-known 3d spatial orientation problem of Gimbal Lock. When the player's ship is at extremely high (or low) angles of attack, that is, the z-angle is at or near +90 or -90 degrees, the normal left-right swiveling movement of the spaceship becomes indistinguishable from movement about the y axis (in this case, roll).

It's not a huge problem, because the player can either tilt further (through the deficient z-angles) or back the tilt off into a more usable range. Initially, though, it can be confusing.

If I really want to solve the problem (and I should, since in the heat of a game you don't want unexpected reaction to your keyboard commands), I may have to use Quaternions instead of Euler Angles to describe the player's orientation in 3D space.