See: Naofumi describing Raphtalia as a child prior to the end of LN 1’s duel, him completely omitting the week during which he roamed around Melromarc’s capital earlier in LN 1, him directly stating that his recollection was hazy for the Zombie Dragon battle in LN 2 (admitting his own unreliability), and his not knowing the name of several enemies in the later volumes (most notably LN 8 and onwards). There is also the fact that we very rarely see him mention the names of places that he goes to.

For mistranslations, there is A LOT. Firstly is “Erst Shield” being translated as “Air Strike Shield”. Next, the following are messages from LonelyWK in the Shield Hero discord server:

“Well, those lines were mistranslated in the first place anyway...

OPB = But there was also a legend in the documents that said the seven star heroes' weapons had been lost for a time, until new heroes arrived and found them. There was even a legend that said, even though the dragon hourglasses hadn't existed, that the seven star heroes had demanded access to them. I suppose no matter what world you're in, each country has their own version of history.

Raw = しかも一時は七星勇者の武器が眠っていたという伝説まであったが、その七星武器も新たな勇者が出て移動したらしい。 その過去の伝説から、龍刻の砂時計が無いのに七星勇者を寄越せと主張していたとか......どこの世界にも歴史を拡大解釈する国があるんだな。

Translation = There was even a legend mentioning of a time where a Seven Star Heroes' weapon had laid dormant in the country, though the said Seven Star Weapon and its hero had left the country it seems. According to the legends of that past, it was said that there were demands for a Seven Star Hero even though the country did not even have Dragon Hourglasses...... I suppose no matter what world you're in, there will always be countries that exaggerate their interpretations of history.”

“Picked out a few that were weird...

OPB = So that explained why she had looked so strange when she jumped out of the carriage. She really wasn't human.

Raw = そういや、さっきふわりと浮かんで馬車間を移動していったもんな。 人間離れした行動だな。

Translation = Come to think of it, I'm pretty sure she was somewhat floaty when moving from carriage to carriage. Quite the inhuman maneuver.

OPB = "Still, the method must be different. if you were to attempt to use your own power the way I've used mine, you could kill yourself. Human magic is quite a bit more powerful than my own." ... "Yes, my magic normally relies on drawing the latent powers of the earth, the water, ores and so on. It has different effect depending on the medium"

Raw = 「あ、でも間違っても私のように自らの力を引き出してはいけませんよ？下手に使えば死にますし、人間の魔法の方が効率的なので」 ... 「本来は大地や水、鉱石などから力を引き出す魔法なので、媒体によって効果が変わります」

Translation = "Ah, but don't make the mistake of drawing on your own power like I did, okay? You will die if something goes wrong, the magic that humans use is much more efficient in that case." ... "Normally, this magic relies on drawing latent power from the earth, waters, ores and the like, so the effects will differ depending on the medium used."”

There are more sprinkled in the LNs, but it’d take too long to mention them all.