Impossible bodies by Hendrick Goltzius and his student Joachim Wtewael [Oot-vayl]:
Mars and Venus Surprised by Vulcan, Getty Museum, c. 1601-1610
Mars and Venus Surprised by Vulcan, Mauritshuis, 1601
I have seen both of these in person and there are much smaller than you would expect. They are also incredibly vibrant because rather than painting on wood, he painted on copper, which better preserves the original color.
Goltzius, Without Bacchus and Ceres, Venus Would Freeze (Sine Cerere et Libero friget Venus), c. 1600-1603
Also something that should be seen in person. Goltzius is known for having done engravings (e.g. below). Here, is is experimenting with pen and oil paint, creating the sinuous lines as if they were made by engraving. The oil is used to add color and highlights, washing most of the painting in blue to give it the feeling of freezing and allowing the warm glow of the fire to stand out. Venus's protruding nipples are echoed by her pearl necklace and she wakes up, giving and receiving various caresses--as the grapes touch her nipple, she strokes Cupid's wing and lightly touches Ceres's shoulder.
Here are some details to show the cross-hatching:
More prints by Goltzius: