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.

Caption 25, Il était une fois: les Explorateurs - 15. Bruce et les sources du Nil

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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...

Caption 56, Banlieues françaises - jeunes et policiers, l'impossible réconciliation?

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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.

Caption 31, Il était une fois: Notre Terre - 9. Les écosystèmes

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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.

Caption 4, Le Jour où tout a basculé - Mes grands-parents sont infidèles

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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 anciens

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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!

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For fun, here's an 80s throwback for you: Sabine Paturel's "Les Bêtises," which was a smash hit in France in 1986.