Lou Schwartz...

Walter while constructing an alibi, called a co worker Lou Schwartz because it was a 'toll call' ('long distance') so there would be a record of it proving that he, Walter, was home all evening. Duh, once beginning a casual 'call to a co worker at home' relationship, it wasn't a guarantee that Lou wouldn't call Walter back with some additional info. he had that he thought would interest Walter or helped him with what he was working on.
If and when questioned by the police, Lou would have likely mentioned how unusual he thought it was that when he called Walter back, he got no answer.
Of course Walter did take a walk to get some dinner, maybe he told Lou he was going out to eat at some point so maybe easily explained away - as long as the times matched.
But its just the kind of thing thing that snags the 'perfect murder'. It reminds me of OJs limo driver showing up early to pick up OJ, something OJ hadn't taken into account.
LOVE this movie, always have, just an observation.