In libraries, when you choose to "Switch to Markdown editor" on a problem, whether it's a new problem (never been saved and has no ID yet) or an existing problem, you see this error popup:

It seems that the markdown editor is trying to call some modulestore API to save changes.
Despite the error, it still seems to work fine.
On the teak branch, a second error appears when saving: "Unable to add component to collection" or something like that. However, you won't be able to test this on the Teak branch unless you use #2098 .