The first postmodernist buildings documented the failure of Modernism. In 1970 an American architect called James Wines set up a group called SITE, which stood for Sculpture In The Environment. They produced radical buildings that commented on Modernism using humour and irony.
SITE was commissioned by a company called BEST Products to design a series of showrooms for them. The Arden Fair Shopping Mall in Houston, Texas (1976-7) looks like a standard Modernist edifice, but it has a fake rift cut into the side [Fig. 1]. We could interpret this as meaning that the orthodoxy of Modernism is breaking up.

Fig. 1 Arden Fair Shopping Mall, Houston, Texas (1976-7)
Another example was the Inside/Outside Building in Milwaukee, Wisconsin (1984). This was another monolithic building, but half the façade is missing [Fig. 2].This exposes the structure, revealing how the building is constructed. Modernism demanded that buildings reveal their structure, but this does it in a very different way - it breaks the façade open.

Fig. 2 Inside/Outside Building in Milwaukee, Wisconsin (1984)
The Peeling Project in Richmond, Virginia (1971) seems like just another banal brick box, but the façade is peeling away from the building [Fig. 3]. Perhaps this is a comment on the superficiality of the Modernist aesthetic.

Fig. 3 Peeling Project in Richmond, Virginia (1971)
Another example is the Ghost Parking lot, in which the tarmac is shaped like a row of cars [Fig. 4]. This challenges the Modernist idea of functionalism: it clearly expresses what it's for, but it can't be used. It's anti-functional. SITE attacked Modernism with humour and irony.

Fig. 4 Ghost Parking lot