This doesn't happen too often, but today it stopped me cold: I was writing a paragraph from Ten Gentle Opportunities and couldn't decide which of two usages was the right one. When there are two ways to say something, I generally have an intuitive sense for which is the more correct way, based on what I've read as much as what I've learned of proper grammar. This is one of the benefits of reading much and broadly.
Today I got stuck between two usages that both felt a little wrong, and both almost precisely the same measure of right. Here's the sentence in question, done the way I learned it way back in the Precambrian:
Carolyn stabbed the End Call button, and rose to go fetch her intern, whosever intern he was.
It sounded a little off. The other way also sounded a little off:
Carolyn stabbed the End Call button, and rose to go fetch her intern, whoever's intern he was.
My (moldy) style books all say "whosever" but I hear "whoever's" a lot more in recent writing. I think what we're seeing here is a usage at the tipping point. In a few more years, "whosever" will become an archaism, and people will look at you funny when you say it.
Is this good or not? I don't know. Language evolves; sound and sense and all that. About the only thing I'm certain of is that I'm old. But I knew that.