In the case of Fisher v Bell, the case revolved around the law of contract - more specifically the requirements of OFFER and ACCEPTANCE.
Under the Restriction of Offensive Weapons Act 1959, it was illegal 'offer for sale or hire... any knife which has a blade which opens automatically.'
The defendant (the Shopkeeper) was charged under the Act for offering a flick knife for sale. He argued that merely displaying the product was an 'invitation to treat' and not an offer. He believed that when a buyer of a knife purchases the product and the shopkeeper accepts the purchase for the product, this at the time constituted to an offer.
The prosecution argued that displaying the knife in the window of the shop with the objective of attracting a buyer/s - constituted an offer of sale, which they believe was sufficient to be criminally liable.
As a result, the Judge found in favour for the defendant, and interpreted the Act to a degree in which displaying a knife did not constitute an 'offer' but only an 'invitation to treat'. The Act was altered to insert the words 'or has in his possession for the purpose of sale or hire' to close the loophole and quite frantically overruled the High Court's decision.
To answer the question - By adopting a narrow interpretation of the Act by the High Court, it meant that displaying a knife was an invitation and no an offer. In my opinion, this literal approach (plain or ordinary meaning) to the law meant that other areas concerning the section could not be considered. E.g. If a shopkeeper displays a knife, and he/she cannot be charged for it, unless someone buys it, leads to a considerable amount abuse and escape from the law. Therefore, the High Court's narrow interpretation does not provide for the law to be fully developed to cover all situations that may arise under this specific section. Thus, the law is flawed and Parliament has made errors in drafting and passing the bill.
Also, altering the Act had the effect of closing a loophole. If this interpretation was not carried out, shopkeepers would be able to sell a knife in their store (by displaying the product with a tag) without being charged for an offence under the Act, unless a customer came into the store and purchased the product with the shopkeeper accepting the offer (by providing a receipt and accepting the money).