A few times I've accidentally activated "Mark Group Read" when I was 
right-clicking to bring up the menu and then select "Get New Headers" 
by left-clicking, probably because I moved the mouse a little or held 
the right mouse button too long.

Since there is no "Are you sure?" dialogue and no undo, this means 
that I unintentionally lost track of which articles I had read.

Similar remarks applies to "Delete Group's Articles".

I therefore suggest:

1.  Move "Mark Group Read" and "Delete Group's Articles" to the bottom 
of the menu, immediately above "Delete Group".  There they won't be 
activated by accident.  Since I use "Get New Headers" much more often, 
I would prefer that command on top anyway.

2. Add an "Are you sure?" dialogue for "Mark Group Read" and "Delete 
Group's Articles".

Personally, I wouldn't mind if those two commands were removed from 
the right-click menu, since I never have needed them.  (They could 
still be kept in the Newsgroups menu in the menu bar, I suppose?)