Это серьёзное последствие ошибки.

Russian usually omits the present-tense form of to be.

So:

• Это серьёзное последствие ошибки
  literally looks like
• This serious consequence of an error

but it naturally means:

• This is a serious consequence of an error

This is completely normal in Russian.

Here это means this or that, depending on context.

In sentences like this, это often works almost like this is or that is, because Russian leaves out is in the present tense.

So Это серьёзное последствие ошибки can be understood as:

• This is a serious consequence of a mistake
• That is a serious consequence of the mistake

The exact English choice depends on context.

Because серьёзное must agree with последствие.

• последствие is a neuter singular noun
• adjectives must match the noun in gender, number, and case

So:

• masculine: серьёзный
• feminine: серьёзная
• neuter: серьёзное

Since последствие is neuter singular, the correct form is серьёзное.

A very common clue is the ending.

Nouns ending in -ие are usually neuter.
So последствие is a neuter noun.

That is why the adjective is also neuter:

• серьёзное последствие

Because ошибки is in the genitive singular here.

The phrase последствие ошибки means the consequence of an error / mistake. Russian often uses the genitive case to express of relationships.

So:

• ошибка = error, mistake
• ошибки = of an error, of a mistake

That is why the sentence uses ошибки, not ошибка.

The form ошибки can be ambiguous:

• genitive singular: of an error
• nominative plural: errors
• accusative plural for some contexts: errors

But in this sentence, it is understood as genitive singular, because it depends on последствие:

• последствие ошибки = consequence of an error

If it were plural in meaning, the structure and context would normally make that clear.

It describes последствие, not ошибки.

So the sentence means that the consequence is serious.

• серьёзное последствие ошибки = a serious consequence of an error

If you want to say the consequence of a serious error, then Russian would be:

• последствие серьёзной ошибки

Notice the change:

• серьёзное modifies последствие
• серьёзной modifies ошибки

In a simple present-tense sentence with это and no expressed verb, Russian normally uses the nominative:

• Это серьёзное последствие ошибки

That is the standard, natural pattern.

You may see the instrumental with a verb like являться:

• Это является серьёзным последствием ошибки

But that version is much more formal and less common in everyday speech.

So for normal Russian, nominative here is exactly what you want.

Yes, but the given word order is the most neutral and natural.

• Это серьёзное последствие ошибки = neutral statement

Russian word order is flexible, but changing it usually changes the emphasis, not the basic meaning.

For example:

• Это последствие ошибки серьёзное sounds more marked and emphasizes serious
• Серьёзное последствие ошибки is just a noun phrase, unless the wider context makes it a full sentence

So learners should treat Это серьёзное последствие ошибки as the safest standard version.

Russian has no articles.

So последствие can mean:

• a consequence
• the consequence

and ошибки can mean:

• of an error
• of the error

Context tells you which English article makes the most sense.

Yes. серьёзное and серьезное are the same word.

In Russian writing, the dots over ё are often omitted, especially in everyday texts. But the pronunciation is still yo, not plain e.

So:

• серьёзное = fully marked spelling
• серьезное = common spelling without the dots

For learners, it is helpful to remember that ё is always stressed.