Hi Michael

Had the same thing happen again yesterday, was working on the attached file, last night, after about 1 1/2 hours working with the same file, I was trying to Boolean Union, the Green and Red solid objects, The objects was created by first making the main "bead" then sweeping the edges (rims).



When I Bool-Union the bead and 1 rim, all OK, 1 new solid, when I union the result and the other rim , ended up with the following defect,







Left them has two separate solids, did some other work , went to bed, this morning open the same file, Bool-union worked fine, first thing I tried.
Went back to through some of the incremental saves, where I first noticed the defect and again it worked fine.

Very strange, last night I did not think to restart MoI and see if that made a difference .