Plot Hole?

The real plot hole: There's only one Stanley Cup. It get's passed around to the new champion team from year to year - yet Leo has had it in his vault for several years? Maybe the NHL hushed up the original thievery and secretly replaced it with a replica, but there should at least be some explanation in the film.

As to the points mentioned above, Sam didn't expect Frank (ex-boyfriend) to tell Hannah that Sam had tipped them off. If he hadn't, Frank might have been back in with Hannah for bringing the trophy back. Alas his conscience get's the best of him and he tells Hannah and Leo, thus she' shows up unexpectedly.

I take it that Sam's plan was to have the other "safe men" steal the trophy back while Sam and Eddie are attending the party with Gale, thus providing a decent alibi. It's not airtight, but Sam's not too bright so it reasons that he would think this was a "perfect plan". Following the quirky logic of this film, it probably would have worked.

This supposed "plot hole" mentioned above is really a screenwriting issue. The script is wonderful, but one of it's central thematic elements is the interconnectedness of characters and environments and the very matter-of-fact way this is played. The way the two sets of safe men repeatedly running into each other, the significance of each of the safe locations - these are good examples. The problem, however, is that this all comes off like setup for a major third act reveal that rationalizes these supposed coincidences. Perhaps there's more to Bernie's choice of safes than there appears to be, and Sam and Eddie's would-be-thefts are part of a larger plan. Perhaps the other safe men were being employed by Leo to ironically equal and opposite ends against Bernie. Perhaps the two sets of safe men are actually brothers (Frank to Eddie and Sam to Mitchell as Frank and Sam meet earlier) thus justifying the serendipitous parallels.

In retrospect, so much track is laid for at least one of those outcomes that it can almost be assumed that such was the case in an earlier draft. Other factors suggesting the first alternative include the fact that just three safes were selected, the fact that Bernie isn't upset when the first two robberies fail as if the cash in the safe itself was never the goal, and the way that he casually mentions that the first two robberies were just "tests". As for the second alternative, realize that Frank and Mitchell are the cities best thieves and Leo, of all possible gangster occupations, just happens to be the cities biggest fence. It's a given that the three have worked together many, many, (many) times before. As for the brothers alternative, consider the the way that Eddie just happens to have a master safe-cracker for a father. The father never shows, and he could have just as easily learned about safe-cracking a different way, but it works as setup for this reveal as it would make sense that Eddie Sr. has another son (Frank) who's a master-safe cracker in the same town as Eddie.

With a few changes, it could have come to light in the end that all of this was true. How about this: Big Fat Bernie Gayle is a top Jewish gangster in Rhode Island who care's about few things more than his son, Bernie Jr., who's Barmitzvah is fastly approaching. Bernie Jr. is a tremendous hockey fan, and it just so happens that an authentic Stanley Cup is secretly stashed in the safe of Bernie's rival Good Stuff Leo - what better Bar Mitzvah gift? The only problem, Rhode Island's best safe men, Frank and Mitchell, are already in Leo's camp. It fits - good thieves do good business with good fences. It just so happens, though, that Bernie remembers Frank's father, a master safe man back in the day, and that he had another son too - what ever happened to him? Veal Chop finds out that the second son is now a frustrated lounge singer with an ambitious (albeit somewhat slow-witted) partner. Bernie, with the Machiavellian genius of a wise old gangster, hatches a plan. If he can trick Eddie and Sam into passing themselves off as safe men, give them a few "test" robberies to cut their chops on, and make them believe that their lives are at stake, eventually Eddie's "safe cracking genes" will kick in and Bernie will have a capable thieving duo of his own to steal the trophy.

Possible sub-plot: Maybe Bernie and Leo are in a "stealing war" during the first two acts - as it turns out, that was Bernie's house that Frank and Mitchell were robbing in scene 1. The plan Bernie set's in motion is also motivated by a desire for retribution, thus what better place to send his new thieves on their first assignment than Leo's house? Even though they fail, Leo is slighted when he finds out about the attempt and retaliates by sending Frank and Mitchell to rob Bernie's office. At the same time, when Bernie finds out Eddie and Sam failed, Bernie figures he better play it safe (pun intended) and make their second test robbery some place he can control so he sends them to his office as well. This is the second robbery. The next day he finds that the safe was indeed robbed and figures that Sam and Eddie finally got it together just as he expected they would. Leo knows it was really Frank and Mitchell who robbed the safe and so he feels satisfied.

Of course, none of this comes to light until the third act. If reveals like this were present, the ending would feel more complete.