The masterful trompe l’oeil frescos on the (flat) ceilings of the Church of Sant’ Ignazio di Loyola in Rome were painted by Andrea Pozzo in 1685. One depicts the apotheosis of St Ignatius—he rises up out of the ceiling into the clouds above. The other presents the illusion of an elegant dome; sunlight shines through a ring of small windows in the cupola. Pozzo likely painted the second one after money ran out for an actual dome.

Image sources: here and here.