Professionally lots, but maybe this one over all others, probably about start Feb 2020 (we work in infectious diseases):
"This is it, the big one, this is the Spanish Flu of our generation" I told a colleague. He turned instantly pale & quietly said "16 million people DIED of Spanish flu."
I flustered and said "I don't know if that will happen, but this is going to be big ."
Personally many, this jumps to mind:
Large Playground in village: finding big locks on gates and warning "Closed due to covid!" signs. Never reopened after March 2020. Garden centre & golf course nearby reopened of course. Places old people want to use, they re-opened, but not spaces children like to use. I have smouldering rage about this.