Some other 'greatest upsets' that I could think of right now ...

 

1.  The abrupt halt of Mongol advance into Europe in the 13th century.  I would say this is among the 'greatest upsets', because it was not a result of someone stopping the Mongols, but the Mongols stopping themselves due to their internal political snag while there were no one in the Continental Europe capable of out-fighting the Mongols at that time.  (Ogodei, their khagan, had passed away and thus marked the beginning of fracturing of the Mongol Empire.)  The Mongols never came back.

 

2.  The abrupt halt of Chinese naval expedition in the early Ming Dynasty.  Similar to the Mongols, they were not stopped due to military reasons, but due to a political one.  (Eunuchs and non-Han officials vs. Han court officials.)  The first European transoceanic attempt was still half a century away.  Not only the Chinese stopped, they soon scrapped their navy.

 

3.  The Chinese intervention phase of the Korean War.  PRC did not keep its intention secret at all.  They have repeatedly threatened to intervene.  Furthermore, once the Chinese started sending in troops (in October 1950), South Korean officers were alarmed by Chinese prisoners taken as they advanced towards the Yalu River.  But all these signs were ignored by McArther & Co.  The UN forces were routed during the worst of the winter of '50-'51 and it went so badly that they had to abandon Seoul.  Among the most extreme swings of generalship of the 20th century -- McArthur was nothing short of awesome only a little while ago when he pulled off Inchon landing.  Once the Chinese were on the loose, he was never able to recover, although his successor, Ridgeway, was able to step in and turn things around just in time.

 

4.  The Russian blunders before and during the Russo-Japanese War (1904 - 05).  Even the prepatory part of this war went badly wrong.  Among other things, they not only assisted the Japanese by deploying their Baltic Fleet to the Pacific (thousands of nautical miles of worth of re-deployment is exhausting as hell!), but also botched by having to discharge their wireless telegram operators.  (These wireless operators were German civilians and they had to be discharged at Madagascar as they refused to be put under military discipline, but the Russians were critically short of wireless operators of their own.)  The Russians also botched diplomatically -- Japan was backed by an alliance with Great Britain at that time, and other major powers like the US was also weary of Russian expansionism.  Additional blunder -- Jewish financiers were also supportive of the Japanese at that time as Russia's pogroms have outraged quite a number of Jewish activists and intellectuals.  The financial support angle turned out just as important as Japan's treasury was rather poor compared to that of Russia.