(with Vavoom 1.26 win32, Doom)
If there are adjoining sectors with open sky texture (F_SKY1) and their ceilings are at different heights, there can be a problem with middle textures used on the 2s line between those sectors.
If the line is upperpegged or there's no upper/lower peg flags at all, the middle texture's frontside and backside are drawn at different heights according to ceiling heights both in front and behind the line. Please, check this.
a short demo level:
http://www.geocities.com/gearlost/misc3 ... _fsky1.zip