If both pizzas have a sticker stating 2 for £4.00 you have been over charges.

If you have picked up one that stated 2 for £4.00 and the other stating reduced to £2.29 you may have picked up two different pizzas with different offers. Sometimes people pick items up from one place and put them down when they see another offer, always check the labels/price ticket.

If it was from one of the large supermarkets you could phone the helpline number usually available on the Supermarkets web page, they can check the price via the bar code. If you have been over charged they will usually send you a voucher to cover the overcharge + a little extra. With Morrisons you have to take the good back to the shop.

It's also a good idea to check the maths on the multi-buy offers, as this photo demonstrates quite clearly...

http://s300.photobucket.com/albums/nn27/interzone55/?action=view&current=IMAG0023.jpg

found all supermarkets display offers on shelfs but not always at tills [tesco,sainsbury,waitrose] got so fed with tesco customer service at their sunbury branch being rude when I pointed out errors each week that we stopped shopping there, at least the other two stores had polite helpful staff when these errors were pointed out.

This is caused by barcodes. The full price pizza would have had a barcode which would have triggered a reduction at the till if a second one with the same barcode had been scanned. The reduced pizza had to have a different bar code attached to the reduced price so that it would scan correctly at the reduced price but this did not trigger the offer. Could be a coding error in the reduced barcode.

Tesco show at the bottom of the list how much money is saved through bogof offers etc. I always check this before leaving the store. This shows whether the correct deduction has been made for multiple purchase offers.