Meiltä on tullut joskus mentyä väärään terminaaliin vahingossa, mutta nyt tarkistamme lentoyhtiön viestin kaksi kertaa.

Yes, meiltä normally is the ablative form of me and often means from us.

But in this kind of expression, meiltä on tullut... does not literally mean physical movement from us. It is part of an idiomatic pattern:

• minulta on tullut tehtyä
• sinulta on tullut sanottua
• meiltä on tullut mentyä

This pattern means something like:

• I have ended up doing
• you have happened to say
• we have ended up going

So meiltä marks the people involved in a slightly indirect way. It often gives a nuance of:

• accident,
• habit,
• lack of deliberate intention,
• or a mildly sheepish admission.

Idiomaticly, on tullut mentyä means has ended up going or has happened to go.

Word-for-word, it is not something you should translate literally into natural English. The important thing is the whole pattern:

• on tullut tehtyä = has ended up doing
• on tullut sanottua = has ended up saying
• on tullut mentyä = has ended up going

In this sentence, Meiltä on tullut joskus mentyä... means something like:

• We have sometimes ended up going...
• We've sometimes happened to go...

It sounds less direct than simply saying we went.

Mentyä is a special participle form used in this idiom. Technically, it is the partitive form of the passive past participle of mennä.

You do not need to memorize the full technical label right away if that is not helpful. The practical thing to learn is the pattern:

• [person in -lta/-ltä form] + on tullut + participle

Examples:

• Minulta on tullut unohdettua = I have ended up forgetting
• Häneltä on tullut sanottua liikaa = He/she has ended up saying too much
• Meiltä on tullut mentyä väärään paikkaan = We have ended up going to the wrong place

So mentyä is not an ordinary infinitive here. It belongs to this fixed construction.

It is impersonal/passive-like in form, but the meaning still clearly points to us as the doers.

The action is being done by the people referred to in meiltä. So the sentence still means that we are the ones who went to the wrong terminal.

Finnish uses this kind of structure when the speaker wants to sound:

• less direct,
• less self-blaming,
• or more like this has happened to us rather than we actively did this.

So it is not a true mystery subject. The doer is understood.

You absolutely could say that.

• Olemme joskus menneet väärään terminaaliin = We have sometimes gone to the wrong terminal

That version is simpler and more direct.

The actual sentence uses meiltä on tullut joskus mentyä... because it adds a nuance like:

• we’ve ended up doing that
• that has happened to us
• we’ve made that mistake sometimes

So the difference is mostly about tone:

• olemme menneet = straightforward fact
• on tullut mentyä = softer, more accidental, a bit more self-aware

Yes, joskus could appear in other places too, because Finnish word order is fairly flexible.

Here it modifies the whole event: this has happened sometimes.

Possible placements include:

• Meiltä on tullut joskus mentyä...
• Joskus meiltä on tullut mentyä...
• Meiltä on joskus tullut mentyä...

All of these are possible, though the rhythm and emphasis shift slightly.

In the original sentence, the placement is natural and keeps the idiomatic chunk on tullut mentyä together quite nicely.

Because mennä expresses movement toward a destination, Finnish uses the illative case here.

• terminaali = terminal
• terminaaliin = into/to the terminal

The adjective must agree with the noun in case and number:

• väärä = wrong
• väärään = to the wrong

So:

• väärään terminaaliin = to the wrong terminal

Compare:

• väärään terminaaliin = to the wrong terminal
• väärässä terminaalissa = in the wrong terminal

The first is destination, the second is location.

Yes, the construction on tullut mentyä already suggests something unplanned or unintended.

But vahingossa makes that meaning explicit:

• vahingossa = by accident, by mistake

So using both together is not strange. It adds emphasis, a bit like saying:

• We’ve sometimes ended up going to the wrong terminal by accident

In other words:

• the construction suggests accidentalness,
• vahingossa states it clearly.

Because Finnish verbs already show the subject through their endings.

• tarkistan = I check
• tarkistat = you check
• tarkistaa = he/she checks
• tarkistamme = we check

The ending -mme means we.

So tarkistamme already includes the subject. A separate me is optional and usually only added for emphasis:

• Tarkistamme = We check
• Me tarkistamme = We check / We are the ones who check

There are two different things happening here.

1. lentoyhtiön
   This is the genitive form of lentoyhtiö and means:

   • the airline's
   • of the airline

2. viestin
   This is the object form used here with tarkistamme.

Because the idea is checking a specific, complete message, Finnish uses the total object:

• viestin

If you used viestiä, that would be the partitive object, which would sound more like:

• some message,
• part of a message,
• or a less bounded action.

So:

• tarkistamme lentoyhtiön viestin = we check the airline’s message
• tarkistamme viestiä = we are checking a message / some message

Kaksi kertaa means two times or twice.

This is a very common Finnish way to express frequency:

• yhden kerran = once
• kaksi kertaa = twice / two times
• kolme kertaa = three times

There is also a single-word equivalent:

• kahdesti = twice

So both are possible:

• tarkistamme viestin kaksi kertaa
• tarkistamme viestin kahdesti

Kaksi kertaa is very common and neutral.

Not necessarily at this exact moment.

In this sentence, nyt is best understood as:

• now
• nowadays
• from now on

It creates a contrast with the earlier habit or mistake:

• Meiltä on tullut joskus mentyä... = This has happened before
• mutta nyt tarkistamme... = but now we check...

So the sense is:

• in contrast to that earlier behavior, we now make sure to check twice

It is natural Finnish.

The second clause, mutta nyt tarkistamme lentoyhtiön viestin kaksi kertaa, is very straightforward standard Finnish.

The first clause is a bit more idiomatic because of meiltä on tullut mentyä, but it is still a normal and useful pattern. Finnish speakers often use this structure when talking about things they have done unintentionally or habitually, especially with a slight tone of:

• it just happened
• we ended up doing that
• we've been guilty of that sometimes

So yes, it is natural; it is just more nuanced than the simplest possible wording.