We transited through LHR yesterday. Still had to go through security - not sure why since we stayed on the air side.

Anyway, signage required us to empty our refillable water bottles. Odd. Thankfully we eventually found a refill station.

The scanners flagged a still sealed can of ginger ale left over from our incoming flight. It was "fine" but she still swabbed it. Shrug.

If you come in from a country that doesn’t fall under the TSA, you have to clear TSA before getting on a flight that does.

The worst I had was in India, flying to the US. Not only was there the normal airport security (despite having come in on a connecting flight from within India), but when I got to the gate (with only minutes to spare), there was a whole TSA check at the gate itself. Bags x-rayed (again), metal detectors (again), guy with a wand (again), the whole deal. Just getting to the gate, I had to show my papers to at least 6 people; every time I turned down a new hallway. That was my far my worst airport experience.

It's super frustrating losing the contents of your water bottle and then having nowhere at all that you can refill it.

Pretty common to have to re-clear security at large airports if you've come from another country, I've had to do it every time when transiting through Dubai for instance.