Blackhorse77 : " The way to keep the pair straight is to focus on this basic difference: than is used when you're talking about comparisons; then is used when you're talking about something relating to time. Than is the word to choose in phrases like smaller than, smoother than, and further than." And don't get me started on your and you're!
@Vinbert : One thing I've noticed is that when I'm on certain sites, I will type something correctly and knowing I did so by checking it and then seeing later after it publishes that it was changed in that pages system to incorrect spelling. ( I literally just typed out incorect, saw the error, went to fix it by adding another r and watched it change to incc instead of adding the r.)