I'm using QGIS 2.2 and I have a polygon shapefile which was created by someone else. I have managed to get the shapefile to a topologically clean state by running v.clean but when I tried to cut the existing polygons to show a corridor (for a new powerline) gaps and overlaps then appear when I run topology. I have 'enable topological editing' crossed and snapping on and avoid intersection. Each time I try to cut the polygons new gaps and overlaps appear in different places.
Instead of cutting out polygons, an alternative is to reshape your polygon (Toggle editing > Edit > Reshape Features) into your desired corridor. The topology should remain intact as no polygons are being removed.