Animals are generally (and perhaps unjustly) considered to be less intelligent than humans, which explains why the French word bête can mean both "beast" and "stupid":
Après tout, c'est bête la guerre.
After all, war is stupid.
Cap. 25, Il était une fois - les Explorateurs: 15. Bruce et les sources du Nil - Part 7
The related noun bêtise can mean anything along the lines of "stupidity" or "idiocy." You can use it in a general sense to talk about "something stupid":
Après les parents, ils me disent, quand ils font une bêtise...
Later the parents tell me, when they do something stupid...
Cap. 56, Banlieues françaises: jeunes et policiers, l'impossible réconciliation? - Part 1
Or you might use it to refer to something more specific, such as a mistake. Une bêtise isn't just any old mistake, but a particularly stupid one:
Vous allez réparer vos bêtises.
You're going to repair your stupid mistakes.
Cap. 31, Il était une fois - Notre Terre: 9. Les écosystèmes - Part 4
Of course, if you tell someone he or she has made a stupid mistake, you could be implying that the person him or herself is stupid. Une erreur is a more neutral word for "mistake" that doesn't connote stupidity:
Elle fait une terrible erreur
She's making a terrible mistake
Cap. 4, Le Jour où tout a basculé: Mes grands-parents sont infidèles - Part 3
The plural bêtises is often used to refer to "nonsense," "mischief," or any kind of naughty behavior:
Arrête tes bêtises.
Stop your nonsense.
Cap. 28, Agnès Varda: Le lion volatil - Part 2
Mais si on fait des bêtises, on sait jamais...
But if we get into mischief, you never know...
Cap. 89, Actu Vingtième: Le Repas des anciens
If you argue with someone over des bêtises, you're arguing over nothing:
Mes enfants se disputent toujours pour des bêtises.
My kids are always arguing with each other over nothing.
When it comes to learning a language, there's no such thing as a stupid mistake. So don't fret if you forget an accent mark or type in the wrong word in a Yabla game—you've just made a simple erreur, not une bêtise!
For fun, here's an 80s throwback for you: Sabine Paturel's "Les Bêtises," which was a smash hit in France in 1986.