I agree Nitpicker (which is a colloquialism for being pedantic, oh the irony!). The occasional slip up is easily done and therefore excused. The issue is more focused towards not even knowing when one is slipping up. If you always say "left" instead of "leaving" because you don't know or care that it is incorrect, then re-education is required. Hence the purpose of this thread.
As Ranga said, standard phraseology serves a higher purpose that being a platform to nit pick. It is intended to reduce any confusion between parties.