Short edges

[micheus]

I just checked the code (long tracking) and it came from the fact that thin faces are discarded if Sum/Longest =< 1.001
Sum = sun of all face's edges
Longest = the most long edge in the face

The reason I got from a comment in the wings_vertex.erl (try_connect_1/4):

%% It is crucial that we reject long thin faces, such as
%%
%% A
%% |
%% |
%% C
%% |
%% |
%% B
%%
%% (where the edges are A-C, C-B, and B-A), since the
%% Connect command for vertices will try many combinations
%% of pair of vertices if the user has selected more than two
%% vertices.

If you use a box 1x1x1 and Inset with the same 0.005 you'll be able to cut/connect them.

Plane Cut is a plugin and it's not using the wings3d's core implementation for cut edges.