Lilian Stanhope addresses Columbo inconsistently, pronouncing his rank as 'Loo- tenant', the American way, and 'Left-tenant' the British way. An English actress of her standing in the 1970s would always use the British pronunciation.
The sign outside the theatre advertises "In memoriam of Sir Roger Haversham". In Latin, "In memoriam" means "in memory of", so the sign translated into English would say "In memory of of (sic) Sir Roger Haversham".
As Columbo sits reading the Evening Echo on a bench, an elderly man is feeding the birds next to him. His position changes noticeably over camera shot changes as O'Keefe walks over and attracts the Lieutenant's attention, from upright to leant over.
In the very first scene, Nicholas stops with his car in front of the theatre. There are road markings in the middle of the street in one shot. In the next shot they are gone. There's also a completely different set of people on the pavement/sidewalk.
At about 80 minutes, Durk is telling Colombo that Sherlock Holmes was perhaps their (Scotland Yard's) most famous detective. This is false for two reasons. First, Holmes was fictional, and Durk would of course know that. Second, in Sir Arthur Conan Doyle's mysteries, Holmes was never in Scotland Yard. He operated independently.
When Frame and Stanhope are strapping a bicycle to the back of their car you can see the same extras walk behind them, first from right to left around the corner, then almost immediately return back around the corner from left to right. No attempt to lose carried bags/coats or umbrellas to blend in a little better.
In the scene where Columbo talks to the actors about the murder in their dressing room after a performance, quoting from a folded newspaper what the weather was, it has a banner headline that includes the words "King" and "Constitution", implying this would be concerning Edward VII's abdication in 1936. The Masthead includes the word "Express" but it is not of the London Daily Express, indicating that maybe this was a prop only, and perhaps the only "English paper" that could be found for the scene.