Well, as for me, darker shades of Trollheimr's flag, as well as bolder white stripes of Fierraria's flag are distinguishable, so I never mixed them up and didn't even realize they are similar lol
As for others, except for Austria-Hungary, I can see how the rest can be mixed up.
I think, some people may mix up Netherlands and Luxembourg just because of not being interested that much in those flags and not putting enough attention to memorize, which one has darker shades and which one has lighter. But maybe they look even more similar for you, so I'm not talking about your reasons, sorry if it sounded like that. I can say more, as kids in school, some may have mixed up Russian flag with Dutch and I remember seeing similarity as well, but now it seems impossible to mix those two because of too distinguishable white in the middle.
Ireland can be mixed up with Italy when they use more saturated orange, I remember Irish flag looking like Italian in ESC 2005 graphics, as well as in JESC 2023 postcard, where they used those almost red basketballs
I can mix up Australia and New Zealand sometimes myself when not putting enough attention, but now I see that Australia has bigger star to the lower left corner, never thought about that lol. And never knew about silver fern flag, looks great
Monaco and Indonesia are literally the same here lol
They have no difference in shades and proportions. But in other sets of flags, Monaco may be narrower and also use slightly different (darker) shade of red and Indonesia may be wider and with brighter red, and that's how they may be distinguished