........I am quite frequently picking younger guys up after I have cleared them to FL60 when they check in "climbing 6000". Altimetry and strict adherance to its conventions was drummed into me in the air force but, clearly, similar emphasis is being missed somewhere along the way. You simply have to be an absolute pedant when it comes to altimetry.....
Say nothing but give them a TCAS or EGPWS alert. That might wake them up.
Agree about RT pedantry. Many folk seem to think it is 'flashy' or clever to abbreviate RT phraseology, but they completely fail to understand that on a bandwidth limited VHF radio link, with atmospheric interference and less than perfect broadcast quality microphones; What you might say into your microphone does not come out perfectly in the headphones of the person receiving. Also, 'your' abbreviation might be completely misunderstood by the person receiving, so it is really important to only use approved and correct phrases.
Embarrasing as it can seem - we have surely all been there - if you cannot understand a clearance due to a bad microphone or different accent or whatever, please check and clarify. In this case, if the crew had simply asked (on the ground) "Just to clarify; are you clearing us to Altitude 2000 feet or Flight level 2 zero zero?" it would have easily resolved the issue. There can be no shame in double checking in aviation.