Seems that the "maxlength" values are greater than the server allows; I tried to post a reply to a topic with the quick reply, and received a "Subject Too Long" error. I then clicked "Post Message" on the full reply page; same thing. I had to manually remove the ellipses from the subject field.

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It's just the stage at which the subject was being escaped. Because of the apostrophe in the subject, once the subject was escaped its' length was actually 51 characters. I've reordered those parts so the values are escaped ready for insertion into the database after the checks on the length have been made.