Twenty-second Amendment to the United States Constitution
Some argue that the 22nd Amendment and 12th Amendment bar any two-term president from later serving as vice president as well as from succeeding to the presidency from any point in the presidential line of succession. Others contend that the original intent of the 12th Amendment concerns qualification for "service" (age, residence, and citizenship), while the 22nd Amendment, concerns qualifications for "election", and thus (strictly applying the text) a former two-term president is still eligible to "serve" as vice president (neither amendment restricts the number of times an individual can be elected to the vice presidency), and then succeed to the presidency to serve out the balance of the term (though prohibited from running for election to an additional term).
Thirteenth Amendment to the United States Constitution
While the Constitution does not provide the President any formal role in the amendment process, the joint resolution was sent to Lincoln for his signature. Under the usual signatures of the Speaker of the House and the President of the Senate, President Lincoln wrote the word "Approved" and added his signature to the joint resolution on February 1, 1865. On February 7, Congress passed a resolution affirming that the Presidential signature was unnecessary. The Thirteenth Amendment is the only ratified amendment signed by a President, although James Buchanan had signed the Corwin Amendment that the 36th Congress had adopted and sent to the states in March 1861.