Reproduction: Create a new email. Put focus into compose area. Click Markdown Toggle button. Check error console.
The following will be seen:
Timestamp: 2013-02-02 10:53:46 AM
Error: InvalidNodeTypeError: The supplied node is incorrect or has an incorrect ancestor for this operation.
Source File: resource://markdown_here_common/markdown-here.js
At this point in time, that line corresponds to this: https://github.com/adam-p/markdown-here/blob/8cf8f2a520b2ecc1816737dc65492e52e491c61b/src/common/markdown-here.js#L365
I'm marking this as low priority, since it doesn't happen once something is typed. And everything seems okay even after it happens. So there's very little actual impact besides an error console entry. (But I am afraid that it may indicate an underlying edge-case logic problem.)