Plot hole: When Renee Russo was reviewing the tape showing the room with the painting to see who stole the painting, and the tape was blank because the heat generated from the suitcase hidden under the bench, why didn't she just back up the tape completely to see who put the suitcase there in the first place? (00:28:30)
Plot hole: At the end of the movie when everyone is standing in the gallery looking at the painting now back in place (with the smoke and sprinklers still dripping) they notice that now there is a new painting missing, the one Thomas stole for Catherine. However, none of the other metal wall coverings open or move except for that small side one revealing the empty frame on the wall. This makes no sense why only that one would suddenly open and none of the others. (01:40:50)
Plot hole: The "titanium-reinforced" briefcase Thomas leaves to block the descending security gate would have had his fingerprints all over it.
Plot hole: How did Catherine manage to show up at the original crime scene at the same time that the detective arrived? He came from across town; she came from Europe, as proved by her "jet-lagged" comment the next morning.
Plot hole: The beach could be reached only by vehicle, but they only had one car and they arrived separately.
Answer: I believe that the Monet that Crown hides in his study is not the one that was stolen, it is a copy that he already had prepared. He can enjoy the copy knowing that the original (with the broken spreader bars) is also in his possession. The stolen original then goes to the forger who repairs the broken spreader bars, and then paints another painting (using water soluble paint) over the Monet, so he can "return" it to the museum 3 days later. It gets more complicated when he discovers that Russo is on to him so he has a second forgery made (even the edges forged to match) over the top of "Dogs Playing Poker." He doesn't know if it will be necessary, but given his research into his new adversary, he concocts this contingency. It is likely that he has many contingencies in place, but the "Monet with a ghost underneath" is the only one we get to see. Of course for my theory to hold water, there must be (or have been) that earlier forgery - unless it has been destroyed.
It's not the forgery that he takes out of the briefcase. Even if it were, he still put the Monet in the briefcase at the museum and would have had to break the frame to close the briefcase, thus also breaking the paint and tearing the canvas. The real answer is that it is just something that couldn't really happen, and the movie people don't want the viewer to notice.