Remembrance day grammar

At 11AM today I happened to be checking out at the supermarket when we stopped for a minute's silence, preceded by someone reading the ode over the PA system.

She said "Lest us forget."

What?

Is this some new-fangled grammar-mangling version I don't know about? Am I wrong to have my editor's hat on at such a time? 

Actually, I was more concerned with picking up the bits and pieces my baby was throwing out of my purse from her perch atop the shopping trolley. Still, I'd like to know who's responsible for this horror.