Skipness - spend enough time searching through fares on enough dates, and every now and again you'll find a fare marked as 'NO TAXES'. Rather than being 1p + tax, it'll have a fare of £5 or £10 with no additional tax.
Rare as hen's teeth around bank holiday weekends, but if someone in yield management made a screw-up, nothing to stop SoberLark taking MOL up on his offer !