I know there are grammar habits that are hard to break when we know they should be otherwise, but I just saw someone confess that there are grammar rules they know and refuse to conform to because they just feel too wrong to obey. That's a little freeing, because I suspect a lot of us have at least one of those somewhere.
I know, for example, that there are a few words I britspell no matter what. "Traveller" will always have two "l"s when I write it, and there can be "mold" on cheese but I pour Jell-O into a "mould." My brain rebels at the alternatives.