I've noticed maybe 10-15 times with 1.8 that the TinyMCE editor doesn't load, under normal circumstances (not using edit/editor, or _main, as in the example below).
While banging on _main, this card pretty reliably fails to load 1-2 of the editors: http://test.dwagn.com/main_syntax_testing
I think this only happens with the new :editor view, and only when it appears among other editors in the same card. Happily that's a bit of an edge case right now.
Let me know if you see it in other circumstances.