“Any decent teleportation machine would also teleport the air that you are teleporting in to back to your initial position" 

This is something I just read in a hypothetical discussion about how loud the noise of the rushing air would be if someone teleported away, leaving a vacuum where they had been. Setting aside my surprise that Tumblr doesn’t have ‘teleport’ (or 'tumblr’ for that matter) in its spell-check dictionary, this got me thinking about the consequences of having a 'decent teleportation machine.' 

The first which came to me was that you could get an indication of where a teleporting person had gone to just after they leave. For example, say you are sitting in a room and the person next to you teleports away. If for example they had teleported to somewhere much colder then where you are then you’d be left for some time with a new cold spot where they departed from. This could of course revolutionise (among many things) air conditioning as a patch of air could be periodically reported to exchange with an area of air with the desired qualities (ie cooler, hotter, cleaner,..) 

Returning to the idea of idea of getting information about where someone went, suppose I was stood next to you and decided to teleport into the sea then prepare to have just over 80 litres of sea water lapping at your feet…

This does make me wonder about what the consequences in the Harry Potter world would have been if apparating had caused an exchange of matter and not just moving people from place to another. Especially if you think about splinching…