A fluke can be a fish, the fin on a whale or a part of an anchor in a nautical context, but it's also a recurring feature of naval warfare in the sense of a stroke of unusual luck.
Land and aerial warfare involves innumerable interactions between actors, weapons, environment and the laws of chance so there's a tendency for things to average out. Fluke events tend to be of local impact, although there are exceptions, of course.
HMS Hood from War at Sea
But naval combats are much rarer than land and sea engagements and comprise many fewer moving parts as well, so each battle is full of unique events. The famous Battle of the Denmark Straits which was fought on this date in 1941 perfectly illustrates this.
While a very popular topic for naval games, indeed, nearly every naval wargame seems to include this fight among the scenarios, it's a hard battle to replicate.
From a strategic point of view, the engagement represented a major British success. Their cruiser patrol meant to detect a German breakout attempt succeeded in finding and following the German raid force. The British countermeasure of deploying a powerful surface action force of battleships also succeeded, with the interception of the Bismarck and Prinz Eugen by the battleship HMS Hood and the new battleship HMS Prince of Wales.
It's true that the two British ships, individually, had flaws that put them at a disadvantage compared to the Bismarck, but together they should have been more than a match for the German ship. Especially when considering that the British had to merely achieve some damage the German raider to force it to abort its raid.
So it was truly an amazing fluke when the Bismarck landed that fatal blow on the Hood that exploded the ship within the first few salvos. While the venerable British battlecruiser had a design flaw, it's also true that that design flaw only represented a vulnerability for a limited amount of time and for a limited set of circumstances of range and geometry. The Prince of Wales problems were less of a matter of luck, being mostly self-inflicted breakdowns caused by the ship's incomplete state. Still, under most circumstances the Hood and POW could be expected to score some damaging hits on the Bismarck before being KO'd and in many cases they will outright defeat the German ship.
Despite the disastrous turn of events for the British on May 24th, the essential soundness of their strategy was borne out by subsequent events. Before being forced to withdraw, the Prince of Wales did manage to land a hit on the Bismarck. While not a particularly damaging hit by battleship standards, it was sufficient to end the Bismarck's raiding voyage and send her heading for France. (The undamaged Prinz Eugen departed to begin a raiding cruise).
A few days later, after being harassed by carrier aircraft and destroyers, the Bismarck was intercepted by another 2-battlewagon British task force that wasn't much stronger than the Hood/POW combo. The HMS Rodney and the HMS King George V had no difficulty pounding the Bismarck into ineffectiveness in a relatively short time, which makes one wonder how things might have been if fate had given the Hood a little more time.