At the diner, Don Draper slides two dimes across the table to Sally for her to make a phone call. Payphones were still $.10 in this era and did not go up to $.20 until the late-'70s.
This season takes place in 1969, but the "Hang in there, kitty" poster wasn't printed until 1972.
Peggy's secretary complains in the break room that her boyfriend sent her a dozen roses and that Peggy mistook them for hers. In every scene with the roses, there are more than 12; at least 13 and possibly as many as 16 can be counted.