New mirror BUG with repro @Jay , for a change : ) I've seen this before but couldn't reliably reproduce it.
What just happened was that I created a new mirror by drag/dropping it onto a child of a Mirror. The newly-created mirrors draw at an impossible indent. In this case the two top-level mirror-parents were in different documents. The left one was the original, and the ones that I dragged in to the right one are originally in the same document as the left Parent.

I WAS able to repro this. The key is that the Mirrored Parent has to itself be a child ("SPOT claim for leg xray" in the screenshot). It doesn't matter where the drag-in mirrors are coming from, although their indent level can cause the resulting 'offset' to be larger or smaller. The prefixes, tags, and email attachment don't impact the behavior.