Finding out why nodes aren't producing output

A Merge node and everything that depends on its output aren't producing output when I send those nodes to one of the two viewer panels:
https://drive.google.com/file/d/1FaC4oA ... 8stge/view

Oddly enough making a new Merge node with the exact same settings does produce output on the viewer, but causes nodes depending on it to be highlighted in red:
https://drive.google.com/file/d/1bZfbcc ... OKc2R/view

How do I diagnose what's happening and how can I fix it?

Your Merge3 node has nothing connected to its Background input, that is why you aren't seeing anything. In your example, you can remove it and directly connect the Other Media node to the Background input of Merge4. If you want the Other Media to be in the foreground, you might have to swap inputs on Merge4, you can do this by selecting the node and hitting Ctrl+T.

D'oh, of course... you're totally right. This was a leftover from a different part of a node tree that I cut out earlier and I didn't realize keeping the Merge node around was kinda silly. Thanks Eric!