The entire episode revolves around Frasier and Niles dating/seducing two women they had just met. However, Niles is wearing his wedding ring the whole time, which would have been a dead giveaway to any woman that he was not "available".
Daphne's reference to "an air bubble in a vein" is a huge old wives tale.
Most intramuscular injections are done using a 3cc syringe or smaller.
The air bubble in a vein is called an air embolus. For an air embolus to be
dangerous it would take a 50cc syringe or more (nearly two ounces of air)
to have a chance of being dangerous.
When Frasier gets his flu shot, he screams so loud that Roz throws off her headphones. Frasier's scream would not have been heard through Roz's headphones because he already turned his mic off after signing off.
When Niles is talking about a stainless steel sink, he mispronounces the word "steel" and says "seal".