Here are a few with just a quick google search. An Excellent article both about cav being stopped by squares and squares breaking. As you can see it is not that uncommon so it is not possible to say a square is invulnerable. I started to add up the number of squares broken and I was well of 25 so here you go a list and descriptions of the broken squares. As you can see some are formed and some are not, and a couple due to rain.
William
http://napoleonistyka.atspace.com/infan ... st_cavalry
Cavalry is formed into two troops (one behind the other -or- in echelon formation)
The square fire on the leading troop, while the rear troop charge before the square
can reload their muskets. In 1809 at Wagram Colbert's so-called 'Infernal Brigade'
(9th Hussars, 7th and 20th Chasseurs), rushed against Austrian infantry formed
in two squares . The 7th Chasseurs was greeted with musket volley and fell back .
Colbert was seriously wounded . The 20th Chasseurs then advanced against the
square that had just repulsed the 7th Chasseurs.The French cavalrymen attacked
and broke the square. (The infantry had emptied their muskets and were in a vulnerable
position.) The other Austrian square was broken by the 9th Hussars .
In 1813 at Hanau, 8 French squares and 18 guns were routed by
20 squadrons of Bavarian cavalry. (See picture of Bavarian cavalryman below.)
In 1814 at the last stage of the Campaign of France , Allies' cavalry
with horse artillery broke 6 French squares and 16 guns (4.400 men)
at Fère Champenoise
Mockern 1813.
Two French squares were attacked by 2 squadrons of Brandenburg Hussars (308 men) .
The infantry fired but the salvo made little impression on the Prussians.They broke and
pursued the French who ran towards own artillery and thus masking their fire.
Battle of Nangis (1814), and 5 Russian squares broken by the French.
On perceiving the advance of the French, Gen.Pahlen sent off his artillery,
followed by four infantry regiments, 9 squadrons of cavalry, two Cossack
regiments and few guns. For more than an hour , the retreat was conducted in
good order . The infantry formed in squares kept up their fire, repulsing several
cavalry charges. But when the French dragoons put to flight the cavalry and
Cossacks, the Russian infantry fell apart.
Generals Wittgenstein and Auvray,who were also present in this combat, were
hurried away in the general rout. The Russians lost 2,114 killed and missing,
and 9 guns. The Revel and Seleguinsk Infantry Regiments suffered the most.
One general was cut down and taken prisoner by the French. (The French cavalry
was supported in these charges by Marshal Victor's infantry and Oudinot's
Young Guard.)
Battle of Montmirail (1814), and 4 Russian squares broken by the French.
Napoleon himself left Montmirail for Chateau Thierry , where General Horn ' s
24 squadrons were ordered to keep the French in check until Sacken's Russian
corps should have passed the Marne River. Mikhailovskii-Danilevskii writes ,
"All at once, the whole first line (12 squadrons) advanced to the attack.
The French cavalry waited till it came to the proper distance, and then routed it.
The fleeing squadrons threw the second line (12 squadrons) into disorder, and
galloped off pell-mell along with them in every direction over the plain.General
Heidenrich's infantry brigade, consisting of the regiments of Tambov and
Kostroma, happening to be at hand, immediately formed square, and kept the
enemy in check by a fire of musketry, but they were ultimately broken ;
the general was made prisoner, and 3 guns were taken."