Once again a link broke. This time when I copy/pasted (CTRL+D) a reflective shader (with unique name). The link that broke was between Lambert and an XYZ shader. Which had nothing much to do with the broken link, I'd say. As it happens I had an XYZ shader outside this container too, so that might have been the trouble. But I can't remember if it had the same name (deleted in the meantime).
If you have any nodes in your project with "TEMP_STRING_OLD" somewhere in the name, Paste and Duplicate might not work properly and links could get broken. It acquired this name due to some bug. I don't know if it happened when you duplicated the Reflective Shader or whether it got this name at some earlier time. Perhaps it was caused by a bug in an older build and was then saved into a project or clip file.
I recommend cleaning up those node names if you ever see them.
Regardless of how and when it happened, the broken links might be caused by the node having this name. That's just one possibility though. There could be another bug here which actually resulted in the node having broken links and acquiring the wrong name. I'm still trying to understand what happened.
Matt
Making progress.. I have found a way to reproduce this...
The "TEMP_STRING_OLD" in the XYZ shader appeared as soon as I CTRL+D'd the reflection node. The link broke at the same time. So, it wasn't there before. I actually saw it happen (big event ;)), while sometimes it happens elsewhere. It was a new file, built from scratch too.