Don't think you're only popping out for 5 minutes to get a package from the post office and that you don't need your cellphone for that.
Or that you don't need some metro tickets or your wallet.
Also, don't, after two years without mistaking them, think it's ok to grab the first set of keys without checking which one they are - you might also want to not have two 'related' keychain for the two distinct key bunch you own (your own keys and the one for your parent's place, both with 3 keys and a plastic macaroon keychain, one round/pink the other green/heart).
That will result in you being locked out without means to call/text your SO or the friend who lives 5 minutes away and has the copy of the good keys.
For the next 4 hours
I'm at least gratefull I had my ID (only because I was picking up something), so I finally went to the library and got a card and borrowed two books. I did a lot of walking, even tried to find an outfit for a wedding this summer.
I'm also glad I had an euro to buy a pack of biscuits. My small umbrella (because of course it rained) and lucky I put a sweater and a scarf.
Too bad the weather wasn't nice or I would have just went to a park. And my movie theater card was in my wallet so no sitting for two movies.