I always wonder this -- I work hard to avoid typos and mistakes, but... being human, I do make them. In this case, it hardly matters -- the point I'm trying to communicate is still made. It's not that I don't care or that I'm lazy or that I'm unprofessional -- I'm just a normal human being like everyone else, who occasionally... misses something. The fact that tr readership is seeing those mistakes doesn't really bother me because most are like me -- they see it, shrug their shoulders a bit, maybe chuckle, and go on.