I had no idea with February or someone mentioned Wednesday earlier blush I’m sure as a child I was taught they had silent sounds. I remember a whole thing on it in Year 1 or Year 2!
It was also I who mentioned Wednesday 
This is the whole issue at hand though: errors that become normalised aren't universally known/acknowledged as 'wrong but widely accepted' but, before long, they become 'fact' to most people. Whether it's teachers, parents or any other adults, once you firmly believe something to be a fact, that is what you will teach any children in your care; why would you do anything else?
Anybody who does know and persists in trying to ensure the survival of the correct form will be considered stupid for 'not knowing' and may even be criticised for confusing people by spreading 'false' information.
I remember when we visited our DS's class for a reading session (Y1 or Y2, iirc) and the teachers had spent time making sweet little learning aids for every table depicting two caterpillars of obviously different lengths - one labelled 'longest' and the other as 'shortest'.
The difference between comparative and superlative has been completely lost to most people - so how are they ever going to teach what they have no knowledge of to children? When did you last hear a parent of two children refer to their 'younger' or 'elder' child? We've still retained "This is longer THAN that one" but not "This is the longer of the two".
I think the subjunctive has gone the same way too. People will accidentally get it right for the vast majority of regular verbs, where it happens to be the same, but when did you last hear anybody say "It's important that you be there on time"?
The normalised error is most commonly apparent when (mis)used in the third person. Have you ever recently heard anybody say something like "I must insist that she make her own way there next week"?