And of course it is not unheard of for a native English speaker to correct themselves if they change their mind about which noun to use. You can imagine being asked to very rapidly verbally classify fruits ("it's an apple, it's an apple, it's a pear, it's an apple,..") that you might well find yourself stumbling like "it's a.. an apple".
This is also why a bunch of words have changed - e.g. apron was originally called napron, but when people were saying "a napron" it got mangled to "an apron"..
It's also not unheard of for native English speakers to use it differently (in speech, at least) as a consequence of regional accents. I would use "a history book", because my accent pronounces the h sound. My wife says "an istory book", because her accent drops the h.
But yeah, it should be easier when you're just dealing with text I guess
