Smoking in public places.
Remember that? I recall standing at gigs with young hooded yoofs chain smoking in front of me. That seems a bit odd now, but not a huge jump. However I also remember the days when you could smoke in restaurants (a bit wrong) and even on planes (mental). My boss told me he flew to NYC on a smoking flight a mere decade ago, though surely it must be a bit longer than that...?
Also, feeding the pigeons at Trafalgar Square. That used to be one of the top 10 things to do in London!! Insane. You wouldn't regard going to India and bathing in it's diseased rivers and contracting a deadly illness a good idea, would you? Hang on...
What was once normal and is now a bit odd?
Eg. Jim Davidson.