I ran into once a awhile ago and forgot about it, but ran into i again today–ingle quotes (apostrophes) need to be escaped (by repeating the quote twice) in messages that contain argument substitutions. Otherwise argument substitution is breaks.
@mogoodrich, are you saying that translators need to know to put an extra apostrophe? E.g. someone translating to French needs to write aujourd''hui instead of aujourd'hui within transifex? That seems like an unreasonable expectation, and something we would need to find a fix for on the code side…