This film is based on the Goerge V Higgins novel of the same name. Higgins also wrote Cogans Trade which the 2012 film 'Killing Them Softly' is based on. The character 'Dillon' appears in both, played by Peter Boyle (The Friends of Eddie Coyle) and Sam Shepard (Killing Them Softly).
When leaving the Bruins game, they are driving through either the Sumner or Callahan tunnel. Their destination was the South Shore so they would not be using this tunnel since it would point them toward the airport (or away from it which would be impossible considering their starting point).
When Eddie and Dillon are at the Bruins/Blackhawks hockey game, a Hawks player is pulled down by a Bruin near the Chicago goal. Clearly visible in the background is the Chicago goalie wearing jersey number 35 (this was Tony Esposito in the 1970's). There is a cut to Eddie and Dillon in the stands and then a long shot showing the subsequent on-ice altercation. The Chicago goalie is involved in this brawl but he is now clearly wearing jersey number 1 (Gary Smith, the Hawks backup goalie in those days).
Eddie 'Fingers' Coyle: Can you imagine being a kid like that. What is he, 24 or something. Greatest hockey player in the world. No four - Bobby Orr. Geeze, what a future he's got, uh?