Hmm, whilist trying to draw a sector about 128 fracunits away from the "red barrier" on both ends in the bottom right corner then trying the same in the top right corner, I got an overflow error. The first sector was 32x32 units big ith default settings (64 ceiling with F_SKY1, 64 floor with FLOOR0_6, 255 brightness and no other specials) and the moment I right clicked or pressed insert in linedefs mode, it gave me the error. Same with sectors mode. To be honest, this has probably already been reported and there's not much use to fixing this but it's something to look into seeing as how I gave you enough details.
Upon experimenting btw, I found that trying to create a large sector anywhere close to the red barrier creates the same effect. Hmm.
EDIT: Unrelated note but if you press Shift after pressing insert to have a "drawing" vertex follow you, it creates a lot of vertex "garbage". Only a graphical bug but a bug nonetheless.