Just before the Grinch and Max return to the mountain through the pipe, Max is missing his collar. It's back on him after they arrive at the dump on the mountain.
When the Grinch is rearranging the mail, a package falls on the ground. In the next shot, the package is gone.
At the point when the Grinch visits Whoville, the color of his eyes occasionally change from yellow to white.
When the Grinch finished loading his sled, he had turned all the Christmas lights off. As he was trying to take off, all the lights were on, but his sled stalled. His second attempt of taking off all the lights were off again.
Since Jim Carrey disliked wearing contacts, some shots were changed post-production. In a couple shots, Carrey's real eye color can be seen.
When the Grinch is taking all the presents from the homes in Whoville, he takes Martha May's ring off of the nightstand. Later she slides down the sack of the presents stolen by the Grinch and hands the Mayor back his ring. She is sliding off the sack because she was retrieving the ring from the sack.
When Cindy is ejected from the garbage chute and lands in the snow, it bounces slightly when she lands on it, revealing a crash mat underneath.
When the Grinch rants about Cindy Lou's trespassing, you can see the outlines of Jim Carrey's actual nostrils through the makeup right after he shouts "The audacity!"
When the Grinch takes off his socks and throws them, it is clear the second sock has a hole in it. However, when that sock lands on the ground next to the first one, there is no hole in it, indicating that a different sock was thrown.
When the Grinch leaves the Lou house after robbing them, there are still lights visible on the inside windows.
When Cindy Lou is at the Grinch's home for first time, she knocks on the door, but the sound is different.
An overview shot of the Grinch building the sled reveals the camera's shadow.