What event, situation or battle was averted which could have lead to a catastrophe if it weren't?
Well, “catastrophe” depends upon a point of view because there are always to sides to consider.
Take, for example, a battle of Vorskla. In OTL defeat of Lithuanian-Poliis-TO force is considering catastrophic but, if you look from a different side, Vitold’s victory could be considering catastrophic for the Horde, making it a vassal of the Lithuanian state. It can also be considering potentially catastrophic for the future consolidation of Russia making Great Princedom of Moscow an
official vassal of Lithuania. Probably it could be if not catastrophic then at least unpleasant for Poland because greatly strengthened Lithuania could break the existing union. So can we said that Yesugei averted a major catastrophe by beating Vitold or that if Vitold won the (different) catastrophe could be avoided?
Or we can take a non-event (you wisely allowed for such an option). In 1812 by
not following the initial plan of campaign the Russians avoided destruction of their armies near the border and eventually won a war. From the Russian perspective, the catastrophe was averted and blahblahblah. From the French perspective, this was catastrophic. However, in a broad perspective with a benefit of a hindsight there can be obvious questions about the general long-term “benefits” even for Russia taking into an account the losses suffered from the campaigns of 1813-14 (which were fought mostly for somebody else’s interests), future problems caused by acquisition of Poland, increasingly reactionary reign of Alexander I, stagnant economy, etc. (including the “legend of invincibility” which ended with a catastrophic CW). Of course, there can be broader speculations about the broader positive/negative results of the averted French catastrophe in 1812: pluses/minuses of the “Napoleonic Europe” (minus Spain) comparing to a generally reactionary system established in OTL Congress of Vienna.