Border control on intra-Schengen (ATH-TXL) flight?
 

I flew ATH-TXL today. Yesterday, I'd received an email from A3 about the temporary reinstatement of border controls on flights from Greece to Germany, but I couldn't find any information about it online including on the list of temporary Schengen opt-outs the EU has online. Lo and behold, there was indeed passport control upon arrival in TXL. Does anyone know why/since when this has been taking place?

late 2015, when the refugee crisis started.

Note that independent of this, ATH always had police control of ID/passport (like french airports too).

I once travelled Germany-ATH and back and found it quite customer unfriendly that there was no ID check at all in Germany so you could have gotten to Athens forgetting your ID, and then you are stuck for your way back. Especially given how greece is physically disconnected from the rest of EU and therefore you could also not try to land route yourself back home without ID.

Greece is not physically disconnected from the EU. It borders Bulgaria.

Yes, sorry for inaccurate terminology:

I don't quite remember when my trip was. Long time ago. Might have been before 2007 when Bulgaria joined EU.
Nevertheless, even as of today, it's not part of Schengen, so you couldn't travel freely without ID. And even when it becomes part of Schengen, im am sure ID border control will stay reality for years to come because of the refugee crisis ;-(