When Arthur tells everyone to not move after finding wire cutters, everyone freezes except Polly. As much as she ran the business and is a very capable woman, she wasn't in France and doesn't have a healthy fear of traps like the rest of the men. Doubles as Fridge Horror when you think of how it must have been in the war to have the usually fearless Shelbys scared stiff.
Tommy having "In the Bleak Midwinter" performed at his wedding. The Shelby brothers quote this poem to remind themselves that every day of their lives after their near-death experience is extra. They've also vowed to one another that they will use it to mark their deaths—in this case, the death of Tommy's philandering bachelor days and of his mingling family with business. And just like that night in France, that "death" never came; Tommy continued as usual, getting Grace killed in the process.
Fridge Horror
In the modern day, we know that regular use of strong opiates can seriously disrupt REM sleep and cause vivid, almost hallucinogenic dreams. Tommy's opium is probably actually making his nightmares worse.
In Episode 4 of Season 6, Mosely suggests that Tommy prove his loyalty to Jack Nelson "physically". Tommy grudgingly returns his Nazi salute and "Perish Judah". He's clearly disgusted, but given the circumstances, this seems a small price to pay, and the scene cuts there. When we return, Tommy's Thousand-Yard Stare and Jack's language suggest that something more degrading may have taken place.
Jack: And while on the subject of import and export, Mr Shelby, in light of our new relationship, and since you have proven your commitment in a most extraordinary way, Boston is now officially open for the importation of your merchandise.
