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Parsoid was failing (missing js/api/localsettings.js), so clicking [Reply] on a topic added a pink "An error occurred. The error message received was: Exception Caught: Failed contacting Parsoid"

Clicking [Preview] leaves the error box on-page, even if [Preview] is successful (restore localsettings.js and restart parsoid). Preview handles errors differently (it puts up an alert) but it seems if you're previewing the same post whose reply failed, the error should be cleared.

Turns out Cancel and a successful [Reply] both remove the errorbox, so this is really minor.