How is this for a POD: In 1808 the HMS Phaeton forced the Japanese at Nagasaki to provide the ships supplies. This greatly riled the Japanese, especially the local diaymo, who mobilized his troops to attack the ships. Lets say he manages to levy his samurai in time, and is able to attack the ships before it leaves harbor, and slaughters the entire crew save a few lucky survivors.
The British hear about this by about 1809 or so, but are preoccupied. But by 1815 they are free to respond, and demand compensation and an apology from the Shogunate. When neither is forthcoming, a task force descends upon Nagasaki, and shells the city, burning it to the ground, and perhaps proceeds to Edo to make the same demands for good measure.
This national humiliation forces a re-evaluation of the policy of isolationism. The Shogun starts making inquiries with the Dutch merchants he allows to trade each year about purchasing Western Arms, and acquiring the services of Western military experts. In exchange for enough silk, the Dutch are happy to provide this. As they had proven repeatedly,the Japanese are fast learners and adapters, and within a decade would be manufacturing their own flintlocks and muskets. However, they would have difficulty creating heavy cannon because they lacked large refineries (this was the case before the Tokugawa Shogunate).
The introduction of Western Technology is a destabilizing factor, and could cause the collapse of the Shogunate, or greatly strengthen it, depending on foreign and local factors.