If you create a very simple user macro (with no macro body) that accepts a text parameter, if the text in that parameter contains a left curly brace ({), it will not render correctly in the editor.

If the text is only "{" it can fail to render entirely, only showing a spinner logo. If you enter something like "test{here" it will only display everything after the {, so "here" in this case.

It renders fine within the macro browser when you Preview it, but when you Save it and go back to the Editor window, that is where it fails to render correctly.

Either there appears there is a problem with code on the server that creates the image thumbnail for the macro placeholder, or something in TinyMCE is not properly handling the character before it sends it up to the server (guessing the latter).