When Nappy Tyler rolls his wheel chair over to the cash register to get change to use the pay phone, he grabs a coin out of the right most slot, which is normally pennies, whereas most pay phones at the time needed a dime.
While Paul Drake is watching the people that are faking heart attacks, the diner owner in his wheelchair is reading a newspaper that is just several sheets of blank paper folded to look like a newspaper.
When Paul Drake (William Hopper) is directed by Jerry Ormond (Benny Baker) to go to Nappy's Taqueria to speak with Nappy Tyler (Dan Seymour), Nappy wheels over to the cash register and appears to take a coin from the nickel slot. He then wheels over to the pay phone and puts the coin into the dime slot. Were the coin really a nickel, it wouldn't even fit into the phone's dime slot.
After Paul is poisoned, Della incorrectly tells Perry that Paul is dying of a heart attack - They both seem to take this at face value, not questioning it despite being involved in a case involving patients dying of coronaries that Perry is trying to prove as murders. It seems completely implausible that at least Perry wouldn't be the slightest bit suspicious.
When Della bursts in to the coroner's inquest and informs Perry that Paul is dying of a heart attack, she says she received a call from Dr Hartlund -- but Dr Hartlund knew from the beginning Paul was poisoned, and did not suffer a coronary...