When Slade tells Oliver that Deathstroke used to be his partner, Oliver acts surprised. However, he already learned this information in a previous episode when he first met Slade.
After Slade Wilson ends his fight with Billy Wintergreen and gets shot by Edward Fyers, the sword that he ended Billy Wintergreen's life with is still in Wintergreen's head; but in the next scene, Slade has both swords in their sheaths on his back.
Diggle explains to Felicity about a security detail he was on while deployed to Afghanistan. He says that they were traveling to Mosul. However, Mosul is in Northern Iraq not Afghanistan.
Heart-lung monitors, both hospital-quality and home versions, give different warning beeps for different problems, including loose leads. A loose lead beep is readily distinguished from a flatline beep specifically so that caretakers know what is not an emergency.
When Slade Wilson is describing the plans to take over the landing strip, he says the guy in the tower is in a P.A.T.C portable air traffic control tower, however the closed captions say P.T.A.C.
When Oliver calls Laurel from the island, you can see Laurel answering to a phone that wouldn't have existed 5 years ago, when Oliver was on the island.