Construction Sometimes hours into a build, it will fail when it no longer allows me to delete the part I have selected. Aftert this, it breaks the entire assembly.

[Blaiseifer]

Built assembly fails when I can no longer delete or unselect a part. It will also do this when I try to delete a fairing in the middle of being edited. 

Video of issue:

https://photos.app.goo.gl/UsBec9GR8H1fdsz6A

[Yang366]

I do agree with you. Usually, closing the VAB and returning solves the issue.

Maybe not related, it seems the game becomes more and more laggy, until becoming not usable at all after this bug.
Best option is to close the game and start again.