Pipes with an east-facing (positive X) connection have a collision box that extends south (positive Z), instead of east. This does not affect the rendered AABB.

I'm able to rest an item on the invisible collision box, and to (barely!) slide an item between the collision boxes of two adjacent pipes. Colored bars in the background represent the orientation of the F3 debug crosshair.

This only affects east-facing pipe segments. I tried north, south, west, up, and down, and didn't notice anything out of the ordinary.

Quark 1.5-127 with Forge 14.23.4.2705 (and, admittedly, a long list of other mods... i gotta make me a quark only instance some time >.> )