It’s not a big deal at all, but I see very intelligent and educated people making this mistake all the time. The frequency even seems to be increasing lately, possibly suggesting that the misspelling is gaining ground due to common usage.
Are people doing really doing this intentionally, as claimed in the second definition of the above link?
I’m not that pedant guy, I promise. I’ve never started a thread like this before, I probably never will again, and I apologize in advance for doing so now.
It’s just a very slight pet peeve, nothing more, and this is meant as a public service announcement. 
Fighting ignorance and all that,
Your friend,
Eleusis
(cue Gaudere’s Law in 3…2…1…)