The Vienna Convention did not create the concept of reservations but codified existing customary law.
Perhaps the most famous and controversial reservations are those taken by the United States when it signed the Convention on the Prevention and Punishment of the Crime of Genocide in 1986.
The reservations taken were: (1) That with reference to article IX of the Convention, before any dispute to which the United States is a party may be submitted to the jurisdiction of the International Court of Justice under this article, the specific consent of the United States is required in each case.
Several nations expressed dismay and disapproval at the reservations, stating that it essentially made the treaty toothless.
The responses included: The Government of Ireland is unable to accept the second reservation made by the United States of America on the occasion of its ratification of the [said] Convention on the grounds that as a generally accepted rule of international law a party to an international agreement may not, by invoking the terms of its internal law, purport to override the provisions of the Agreement.As concerns the first reservation, the Government of the Kingdom of the Netherlands recalls its declaration, made on 20 June 1966 on the occasion of the accession of the Kingdom of the Netherlands to the Convention […] stating that in its opinion the reservations in respect of article IX of the Convention, made at that time by a number of states, were incompatible with the object and purpose of the Convention, and that the Government of the Kingdom of the Netherlands did not consider states making such reservations parties to the Convention.
[…] As the Convention may come into force between the Kingdom of the Netherlands and the United States of America as a result of the latter withdrawing its reservation in respect of article IX, the Government of the Kingdom of the Netherlands deems it useful to express the following position on the second reservation of the United States of America:
Moreover, any failure by the United States of America to act upon the obligations contained in the Convention on the ground that such action would be prohibited by the constitution of the United States would be contrary to the generally accepted rule of international law, as laid down in article 27 of the Vienna Convention on the law of treaties (Vienna, 23 May 1969)The Government of the United Kingdom have consistently stated that they are unable to accept reservations to article IX.
It creates uncertainty as to the extent of the obligations which the Government of the United States of America is prepared to assume with regard to the Convention.The problem with inadmissible reservations happens more often with human rights treaties.
…Because of its special character of a human rights treaty, the compatibility of a reservation with the object and purpose of the Covenant must be established objectively, by reference to legal principles, and the Committee is particularly well placed to perform this task.This has been criticized for the reason above-stated regarding option three.
[7] Because of a high number of reservations against human rights treaties, the International Law Commission (ILC) has, since 1994, included the topic in its work program.
The ILC was asked to check if the VCLT would have to be changed with reservations against human rights treaties.
However, when a monitoring body is established by a human rights treaty, it is allowed only to comment on or make recommendations about reservations.
The fact that the monitoring body can comment upon the admissibility of reservations has no effect on the principle of state consent.
Further, the ILC stated that a guide to practice should be made consisting of guidelines to clarify certain problems in the VCLT concerning reservations.
States welcomed this suggestion, although it must be added this guide to practice will have no binding legal force.