I am new to this - but retested my issue in a completely new installation and I consistently get the same issue.
Latest 3.4.2, Create an empty Page using Showcase Template.
Set the Static Home Page to this new Page.
Create a Standard Post and make it sticky (so it will apear at top of the Showcase Home Page).
It does appear, but the background of the post (white) stick out to the left, as if it is not lining up properly.
Did I do something wrong - or is there a style bug within the showcase template?