Also 'of' and 'off', 'Your' vs 'You're', 'to' vs 'too'.
It never ceases to amaze me that printed signs - not hand-written, but professionally printed signs - can state "Smile, your on camera".
I was behind a van the other day emblazoned with the logo "No job to big or to small".
And I've previously mentioned (on this site) seeing a sign-writer's van which stated, in large lettering on the back, "all work fully gauranteed".
Now I know people aren't perfect, and the fact that the majority can read and write at all is great. But these signs are printed by professionals - does no-one read them first?