Other mistake: Pay attention to one of the early innings of the baseball game as one of the Angels' players hits a home run. Though probably intentionally set up this way as to avoid any problems of "logo patents" and such with the real-life MLB California Angels team, he is wearing a helmet without the team logo on it. Also, it is at Dodger Stadium. (01:07:35)
Other mistake: While Frank Drebbin is saying the words "dew glistening on the grass" you can clearly see a woman in the audience at the bottom of the screen repeating every word he says. (01:19:55)
Answer: Because Nordberg was working undercover in Ludwig's organization; not knowing this, the snitch would only see him working with Ludwig's crew, and would thus believe he was dirty.