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.
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.