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.Play Caption
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...Play Caption
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.Play Caption
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.Play Caption
The plural bêtises is often used to refer to "nonsense," "mischief," or any kind of naughty behavior:
Arrête tes bêtises.
Stop your nonsense.
Mais si on fait des bêtises, on sait jamais...
But if we get into mischief, you never know...
Caption 90, Actu Vingtième - Le Repas des anciensPlay Caption
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.