I think it was Andrew Lambert that posited that the sending of a British fleet to meet with the French at Besika Bay (mouth of the Hellespont) in June 1853, at the cusp of the Russian invasion of the Danubian Principalities, was an unnecessary move that OTL triggered the Crimean War. Besika Bay was not within operational distance of Constantinople given the naval technology of the time; the British-French gesture was meant to be purely symbolic rather than a military commitment. However, such a gesture emboldened the Ottomans both to take a more uncompromising stance against the Russians, and also to move their fleet to more forward positions, resulting in the Russian attack at Sinope that made it politically unacceptable for either France or, more importantly, Britain (since France was not likely to fight Russia alone) to back out of the fight.
Without the fleets at Besika Bay - admittedly unlikely since that would have been the likely response for a Near East Crisis - diplomatic efforts might have been enough to localize the Crimean dispute purely into an issue regarding the Danubian Principalities (which would not have led to British/French intervention), rather than a general war about the survival of the Ottoman Empire itself.