If you try to place a block in your main hand at the same time as switching it to your off-hand, you will not place the block, despite it being a valid action in either hand. This causes a very subtle lighting glitch as well.

If you have a block in either hand, it will appear to place the one that was in your main hand and make that sound, then immediately switch to the other.

The lighting glitch seems to have been fixed as of one of the 1.20 snapshots (probably whichever one changed the lighting engine) but the rest of the bug is still valid.