Deliberate mistake: When George spies on his hedonistic neighbor down the hill with his telescope, the telescope is always level when it actually should have been pointing downward. Same when his neighbor spied on him - his telescope was always level also when it should have pointing upward.
Continuity mistake: During the seduction scene, as George and Jenny attempt to make love to Ravel's "Bolero", the record gets stuck. George throws a pillow at the record player, closing its lid and "unsticking" the record. But when George later turns off the record player, the lid is open again.