What was caesars first mistake in dealing with the senate

The crowd of traitorous senators and a bunch of hangers-onsurround Julius Caesar just outside the Capitol. Decius, a traitor, offers a "suit" or a request from Trebonius to Caesar. After a vague but ominous interaction between Caesar and the soothsayer, Artemidorius pleads with Caesar to read his suit (letter) first, as it's dearest to Caesar.