I suppose it depends whether you knew about something in the first place and how long your memory is, but Zaire might qualify as a case of forgotten semi-presidentialism.

Some of my documents are not with me, but Zaire adopted a new Constitution (and a new name for the country) in 1971. As far as I remember, the 1971 constitution was, nominally, extant until 1997. There were numerous amendments during this time and at times there were competing constitutions issued by other participants in the armed crisis.

Anyway, in 1977 the position of prime minister was created. At that time, though, and thereafter, Zaire was officially a one-party state under the rule of President Mobutu Sese Seko. Constitutionally, therefore, Zaire was not semi-presidential in that the president was not selected on the basis of a direct competitive election.

In 1990 the one-party state came to an end, the constitution was amended and Zaire became semi-presidential.

The text of the constitution is available here. It is a classic French-style semi-presidential constitution, though the powers of the president are greater than in the French version largely because the constitution was amended rather than written anew.

In May 1997 President Mobutu lost power to Laurent-Désiré Kabila. President Kabila issued a decree that abolished the post of prime minister.

There is a sense in which Zaire was in such a state of war and near anarchy for part of the period 1990-97 that it is somewhat misleading to say it was constitutionally semi-presidential for at least some if not all of this period. For example, Polity has classed Zaire/Congo-Kinshasa as anarchic (-77) from 1992. However, to the extent that President Mobutu operated under his constitution and the constitution was semi-presidential, then Zaire should be classed as semi-presidential for the period 1990-97.

Obviously, with the passage of the new Constitution in 2006 the Democratic Republic of Congo has become semi-presidential again. Prime Minister Gizenga, the first PM under the new semi-presidential system has just resigned. News about his as-yet-unannounced successor will be posted here soon.