Continuity mistake: When Lt. Snyder breaks the phone booth window with his gun and points it at Hooker, Hooker's hat falls off. Yet in the next shot over Lt. Snyder's shoulder you can see that Hooker's hat is back on his head.
Visible crew/equipment: When Robert Redford is being chased by Lt. Snyder, he jumps from a high place to the ground. One can easily tell it's not Redford but a stunt double.
Chosen answer: Snyder only knew that Erie was friends with Hooker, and he wasn't on Lonnegan's payroll, just a run-of-the-mill corrupt policeman, so he didn't know Erie was involved. The Courier barely saw Erie and so may not have been able to identify him as well as he did Hooker and Luther, so Erie was unknown to Lonnegan.