Baliurel:[First of all: Warning, I'm using here a foreign language, sorry]

My mother tongue has no accusative. But I feel comfortable with the accusative ending from the beginning of my Esperanto knowledge. I forget the accusative sometimes, of course, I'm not perfect, but there is something curious about how the grammatical error pops up. It's (generally) that way: making long sentences or/and speed, but just in the adjective(s), not in the substantive.
And not only the accusative but plural too!

I mean, my mistake specimen is that I unconsciously save -j and -n marks, adding them only once, at the end of the information pack.

(But don't be afraid, kara samideanoj, that's just a little little little bit of times. And despite the fact that there is no vaccine, it's not contagious)

Someone has or had the same curious symptoms?

I remember that when I was a beginner, I had a hard time getting out of my head the notion that -n was the plural ending. I think this was because of an odd Spanish influence; I'm not a fluent speaker of Spanish, but I know it enough to get at least the gist of most Spanish text I come across. I say the influence is odd, because -n is not the plural ending for Spanish nouns, but the plural verb forms usually end in -n. For example, "Ella está" - she is, and "Ellas están" - they (fem) are. So this may have been part of what influenced me to think of -n as the plural rather than accusative ending. But with practice, that little kink got worked out .

Tempodivalse:I noticed that even proficient (even native) Esperantists will occasionally forget to add the accusative -n to a direct object in fast, spontaneous speech (or in fast typing, say on IRC).

Tut tut! I regret to say that Esperantists are also capable of the opposite slip - adding the accusative where it's not required. We can speculate that if the language had been designed differently, errors might be less frequent, e.g using na before an object instead of the suffix
-n, but that would be a different language.