Firstly you must know what is a BUG in Duke Nukem 3D. It's a place where you can go from a room to an other (even with two remote places), by rushing at a wall or an oject of the map... for example, in Hollywood Holocaust, there's a bug behind the doors of the cinema. With this bug, you're able to go directly to the first floor, just behind the red door ! This bug is explained in the BOARD : see the link above ;-)

 If you want to understand WHY and HOW is there any bugs in Duke Nukem 3d just GO HERE !

About fifty bugs have been discovered with a cool tip, which allowed me to easily find a lot of bugs : edit the user.con file and change the runningspeed (set 60200 instead of 53200).So you'll always run as if you were using steroids, and you can fastly rush at any wall (especially when you "feel" a bug) !Go ahead, run through your favorite levels to find new bugs ! But in my opinion, all bugs have been discovered in the first and the third episode, or very few are remaining !