I've come across several references to these illusions and I think I've tracked down the original post by Arnold Zwicky at Language Log. In short, the former illusion is that a particular solecism or pet peeve is very common, when in fact it's not; you just happen to notice every instance because (usually) you don't like it. The latter, of course, means you think that something you've only noticed recently has only arisen recently, when in fact it's been used for a lot longer. Much of this accounts for allegations of declining standards of education in schools, as in "it didn't happen in my day".

Here is the LL article.