Fue un error ridículo.

Both come from ser, but they are used differently.

• fue = preterite: it presents the mistake as a completed event or a specific instance.
• era = imperfect: it describes what something was like in a more background or descriptive way.

So:

• Fue un error ridículo = It was a ridiculous mistake / That turned out to be a ridiculous mistake
• Era un error ridículo = It was a ridiculous mistake in a more descriptive sense, often setting the scene or giving background

In this sentence, fue sounds natural if you are talking about one particular mistake as a finished fact.

Fue is the third-person singular preterite of ser.

Here it means:

• he/she/it was
• or that/this was

In Fue un error ridículo, there is no stated subject, so in English you often supply one from context:

• It was a ridiculous mistake
• That was a ridiculous mistake

Because error is a masculine noun in Spanish.

So it takes:

• un error
• el error
• un grave error

Even though many English speakers may expect nouns ending in -or to be obvious only from memorisation, error is simply masculine and must agree with masculine articles and adjectives.

In Spanish, adjectives often come after the noun.

So:

• un error ridículo = a ridiculous mistake

This is the most neutral and normal order.

If you move the adjective before the noun, it can sound more literary, emphatic, or stylistically marked:

• un ridículo error

That is possible, but it is less neutral. In everyday Spanish, un error ridículo is the standard choice.

Because it agrees with the noun error, which is masculine singular.

Agreement in Spanish means adjectives usually match the noun in:

• gender: masculine/feminine
• number: singular/plural

So:

• un error ridículo = masculine singular
• una idea ridícula = feminine singular
• unos errores ridículos = masculine plural
• unas ideas ridículas = feminine plural

The accent mark shows where the stress goes:

• ri-DÍ-cu-lo

Without the written accent, Spanish pronunciation rules would lead you to stress a different syllable, so the accent is necessary.

It also helps distinguish the correct spelling. In standard Spanish, this word is written ridículo.

Yes, often it can.

In context, ridículo can mean:

• ridiculous
• absurd
• laughable

So Fue un error ridículo could mean something like:

• It was a ridiculous mistake
• It was an absurd mistake

The exact nuance depends on context and tone.

Yes, but the meaning changes slightly.

• Fue un error ridículo = It was a ridiculous mistake
• Fue ridículo = It was ridiculous / He was ridiculous / She was ridiculous

Without un error, the sentence becomes more general and depends much more on context. The original sentence is more specific because it tells you exactly what was ridiculous: the mistake.

Spanish often leaves out subject pronouns when they are not needed.

So instead of saying:

• Eso fue un error ridículo

you can simply say:

• Fue un error ridículo

The subject is understood from context. This is very common in Spanish.

Not directly in its current form.

Fue un error ridículo refers to an error. The adjective ridículo is describing error, not a person.

If you wanted to describe a person, you might say:

• Él fue ridículo = He was ridiculous
• Ella fue ridícula = She was ridiculous

So in your sentence, the thing being judged is the mistake.

A careful pronunciation would be roughly:

• fwe oo-n eh-RROR ri-DI-ku-lo

A few useful points for Spain:

• fue is one syllable: fue
• error has a strong rolled or tapped r sound in the middle because of rr
• ridículo is stressed on dí
• In much of Spain, the c in ridículo is pronounced like the th in thin, because it comes before u? Actually here the c is before u, so it is a normal k sound: ri-DI-ku-lo

So the most important stress pattern is:

• FUE un error ri-DÍ-cu-lo

Not in normal English.

Spanish can omit the subject, but English usually cannot. So the natural translation needs a subject such as:

• It was a ridiculous error
• That was a ridiculous mistake

So while the Spanish sentence has no explicit subject, English normally requires one.

A little, yes.

Error in Spanish is very common and often corresponds to both:

• error
• mistake

In English, mistake often sounds more natural in everyday speech, while error can sound more formal or technical.

So depending on context, Fue un error ridículo may be translated as:

• It was a ridiculous mistake ← often the most natural
• It was a ridiculous error ← also correct, sometimes more formal or technical