I feel your pain... And yes, I know, rearguard action and the rest, and sometimes it's not grammar so much as spelling. Affect/effect does annoy me, and my reaction to the lay/lie thing is usually, yes, to say 'you're not a chicken!'
Every time I see 'ect' for 'etc' I think of Molesworth.
Discreet/discrete puts my hackles up. Not the same word. And prostrate/prostate, although when people get that one wrong, it usually makes me giggle.
The blood pressure rising one, though, I think, is 'them' for 'those'. I was actually guilty of a major etiquette fail once while waiting to collect a child from school; somebody with an apple tree in their garden had set out a table with bags of apples for sale to benefit the PTA, and a parent beside me commented "ooh, I think I'll have some of them apples." At the time I was fighting this battle with one of my children, and my response was both loud and automatic: "THOSE apples!"
Loud, automatic and mortifying - and there's no way to apologise without making it worse.
Oh yes, and on a 'light blue touchpaper and run away' note, I give you...