The painting opposite of Mona Lisa is the The Wedding at Cana. It's one of the most impressive paintings in Louvre (I think it's the largest). I have wondered if they placed it there as a joke.
There is over 100 people in the painting being busy and only person looking outward directly towards Mona Lisa is Jesus looking surprised. Mona Lisa is looking directly at Jesus and smirks.
Hah, I will now believe that this was indeed placed there as a joke.
There are actually a few more people than just Jesus looking out towards Mona Lisa, some with amusing expressions. It is just a rather busy painting, so it can be hard to notice.
There is over 100 people in the painting being busy and only person looking outward directly towards Mona Lisa is Jesus looking surprised. Mona Lisa is looking directly at Jesus and smirks.