Contract formation rules on modified acceptance have been heavily influenced by the so-called favor contractus principle, as opposed to the traditional ‘mirror-image’ doctrine. This is particularly so where the legal solutions of international and European contract law are concerned. The chapter scrutinizes those rules by examining them in terms of practical application and comparative legal background. It is found that the predominantly accepted legal solutions force the finding of a contract by imposing legal fictions that, in the view of the author, are not in line with the still relevant underlying principles of contracts and consent. Emerging concepts like platforms, smart contracts, AI, etc. also seem to reaffirm the ‘mirror-image’ doctrine. The chapter ultimately claims for the restatement of the ‘mirror-image’ doctrine, but rejects the so-called ‘last shot’ rule.