When Elaine is talking to Jerry in the park, in one shot she is drinking from a can of Pepsi. In the next shot the can changes to Diet Coke, then back to Pepsi.
Bette is wearing #15 in the baseball game. When Gennice shows up at Jerry's place after the game she's also wearing #15. Well, she's her understudy, so of course she's be wearing it. It's a gag, not a mistake.
When the understudy is crying after the Bette Midler incident was heard on the news; You can see George about to leave. In the next scene, When Jerry is consoling the understudy (His Girlfriend), you see George sitting beside him.
When George runs over Bette Midler while scoring the winning run, you can see someone catching Bette's stunt double as she flies backwards. In the next cut, she is lying unconscious against the fence giving the impression that she hit the fence after the blow. There is no sign of the person who caught her in the previous cut.
In the Korean Nail salon scene, as George's father is speaking and yelling at the Korean manicurists, the camera cuts to the back room, where the father's former Korean girlfriend is sitting. The background shows Chinese, not Korean. (However, due to centuries of Chinese predominance in Asia, the Chinese language still plays a very prominent part in Korean culture. Most educated Koreans can read some Chinese, and Chinese is often used in calligraphy.)
While Kramer was ordering her food in the hospital , Bette asked him to get her a black & white cookie. Kramer hung up the phone and told Bette that they did not have any. Kramer would not have known whether they had such a cookie because he never requested the cookie while ordering the food.
When George runs into Bette, you can see (in slow motion)that there are stunt doubles used for both Bette Midler and George (Jason Alexander).
From the back camera shot of Kramer running around to icy vendors, you can clearly that it is a stand in for Kramer(Michael Richards).
When Kramer sings to Bette Midler, "you are the wind beneath my wings", his voice is clearly dubbed.
As George runs into Bette Midler and knocks her out, you can clearly see a person wearing a blue/black shirt who catches her. In the next shot though, she is lying against the fence (which was not anywhere near where she would have landed) by herself.
When George rounds third coming home after the camera cuts you can see George's stunt double in the shot after he collides with Bette Midler.
When Elaine is talking to Mr. Costanza in the coffee shop, he mentions the name "Sun Yung Moon." The person referred to is "Sun Myung Moon." This is possibly a deliberate error consistent with the character. (Although it's also possible that "Yung" is the correct pronunciation, it isn't likely, because the written Korean language is ideographic, not phonetic, so silent consonants probably wouldn't occur in English transliterations.)