I had to gate check a bag on a BA flight AMS - LHR and fortunately I had put a (now defunct) bag tracker in it as I handed it over. I requested that it was sent to LHR only as I needed it on my next flight from LHR to HKG. At LHR the bag did not appear and the baggage staff told me it hadn't been loaded at AMS. I didn't believe them because a) it was gate checked and I would have been astonished if it hadn't made it on board and b) my tracker was adamant that it was at LHR. I argued my way up the food chain until a senior baggage person finally believed me and sent a lackey behind the scenes.... sure enough the bag appeared minutes later.

What astonishes me here is how appalling BA's baggage tracking can be, they seem to make stuff up rather than use their technology... sigh!