The battle of Durnstein (11 November 1805) saw an isolated French force north of the Danube come close to being destroyed by a much larger Austro-Russian Army, before French reinforcements saved the day.
In the aftermath of the Austrian surrender at Ulm Napoleon advanced east along the Danube, hoping to catch the Russian army of General Kutuzov before it could escape to the north bank of the river. Most of Napoleon's army was on the south bank, although Mortier's VIII Corps was posted to the north bank. Mortier's corps was new, and was made up of three divisions taken from other corps. General Honoré Gazan's division was moved to Durnstein, on the north bank of the Danube, while Dupont and Dumonceau moved east to join him. Meanwhile on the south bank Murat ignored the Russian move across the river and decided to continue east to capture Vienna.
On 9 November the Russians and Austrians completed a crossing of the Danube at the neighbouring communities of Mautern and Krems. This left Gazan in a very vulnerable position, only three miles to the west of the main Allied army and (at least) temporarily totally isolated. Kutuzov realised that he'd been given a chance to destroy Mortier's corps in detail, starting with Gazan's isolated division.
Kutuzov sent 35,000 troops to attack Gazan's 6,000, but he split his force into five columns with the intention of attacking the French from several directions at once. This dispersion of effort probably played a part in the Russian failure to defeat Gazan's division quickly. The French held their ground for several hours, although Gazan did prepare to ship his guns across the Danube to save them from the Russian.
Gazan was saved from defeat by the arrival of Dupont's division. Although the Russians still outnumbered the combined French force, they didn't press the attack and instead withdrew. By this point Gazan had lost 1,700 dead and wounded and 1,300 prisoners, half of his total strength, and must have been close to defeat. Russian casualties aren’t recorded, but are estimated to have been 3,000-4,000.
Although the Russians had failed to achieve their objectives at Durnstein the defeat didn't have any longer term impact. Kutuzov resumed his march north, eluded the French even after they crossed the Danube in greater numbers (Bagration fighting another delaying action at Hollabrunn (15-16 November 1805), and successfully joined up with the Tsar and Buxhowden. On 23 November Napoleon was forced to call a halt to the pursuit to let his troops rest. He now looked to be in a vulnerable position, but on 2 December fought and won the battle of Austerlitz, one of his greatest battlefield victories.