N3 — outside your level filter

万一まんいち

まんいちman'ichiN3formal

savor + (past) + in the unlikely event — bad, unlikely, but planned for anyway

man'ichi ajiwatta ra

in the unlikely event — bad, unlikely, but planned for anyway

meaning: in the unlikely event of savoring

万一まんいち味あじわったら、こちらの番号ばんごうまでご連絡れんらくください。

man'ichi ajiwatta ra, kochira no bangou made go renraku kudasai.

Please call this number in the unlikely event of savoring. formal

万一まんいちの場合ばあいに備そなえて、非常用ひじょうようの水みずを用意よういしてあります。

man'ichi no baai ni sonaete, hijouyou no mizu o youi shite arimasu.

We keep emergency water on hand in case the worst should happen. neutral

All examples5

in the unlikely event — naming a bad case so you're ready

万一まんいち遅おくれたら、こちらの番号ばんごうまでご連絡れんらくください。
man'ichi okureta ra, kochira no bangou made go renraku kudasai.
Please call this number in the unlikely event of running late. formal
万一まんいちの場合ばあいに備そなえて、非常用ひじょうようの水みずを用意よういしてあります。
man'ichi no baai ni sonaete, hijouyou no mizu o youi shite arimasu.
We keep emergency water on hand in case the worst should happen. neutral
万まんが一いちのことがあったら困こまるから、コピーを取とっておこう。
man ga ichi no koto ga attara komaru kara, kopii o totte okou.
It'd be a problem if anything happened, so let's make a copy. casual

even in the unlikely event — the bad case won't matter anyway

チケットを万一まんいち忘わすれても、受付うけつけで再発行さいはっこうできますのでご安心あんしんください。
chiketto o man'ichi wasurete mo, uketsuke de saihakkou dekimasu node go anshin kudasai.
It can be reissued at reception even in the unlikely event of forgetting your ticket, so please rest assured. formal
万一まんいち間まに合あわなくても、次つぎの便びんがありますので大丈夫だいじょうぶです。
man'ichi ma ni awanakute mo, tsugi no bin ga arimasu node daijoubu desu.
Even if by some chance we miss it, there is another service, so it will be fine. neutral

万まんが一いちmangaichialternant
the fuller form, more common in speech. Just as formal (万まんが一いちのことがあってはいけない).

The case must be one nobody wants (×万一まんいち当あたったら嬉うれしい is
wrong). A lucky win like this uses もし instead.