I’m watching last night’s episode of NCIS where McGee and DiNozzo are out in the woods looking for a dead body. McGee is this little expert Boy Scout.
So why, in a previous episode, did he get terrible poison ivy? In that episode he didn’t know what to do in the woods.
I hate it when they forget stuff like that.
I don’t watch NCIS, but - is McGee at all the type of character who might react to having had a terrible case of poison ivy by compulsively researching any kind of plant he might run into out in the woods and memorizing everything he can, especially whether they have any dangerous or noxious properties? 
Well, IIRC in early episodes Tony misses several clues that have to do with movies, and in later episodes he’s a movie trivia buff, so it seems normal for them to forget that sort of thing - or at least, revise the character’s history when it is necessary to do so.
The same reason in Sherlock Holmes, Dr. Watson has his middle initial change and his bullet wound doesn’t stay in the same place…It’s fiction.
I should point out that in the same series you are quoting, early on Dinozzo knows nothing about movies and later he is a walking IMDB.
I think after the first couple of seasons, when Ziva replaced Kate, they kind of took advantage of that change to rewrite the history of several of the characters, most noticeably DiNozzo and McGee.
The inability of this show’s writers to keep a bible on their characters is frustrating as heck. The movie thing for Tony and the abilities in the outdoors for McGee, but Gibbs is the worst. They changed how many wives he had, interchanged wives 2 and 3 (or 3 and 4) by name a couple of times, his length of service in NIS/NCIS isn’t consistent, how long he’s known Ducky has changed substantially after the Shannon/Kelly revelation. It wouldn’t be too hard to make sure that salient bio points are kept consistent for the characters but they can’t seem to be bothered.
Just recite the MST3K Mantra a few times, and all will be well…