about error handling mechanism

Hi!
I was looking into the postgres error handling mechanism, and the
documentation states that the present mechanism is primitive.

I quote "whenever the parser, planner/optimizer or executor decide that a
statement cannot be processed any longer,
the whole transaction gets aborted and the system jumps back into the main
loop to get the next command from the client application."

Also, it states " it is possible that the database server is in an
inconsistent state at this point so returning to the upper executor or
issuing
more commands might corrupt the whole database"

Since postgres aborts the transaction compeletely, why would it be ever
left in an inconsistant state in such an event?

Also , I want to get someone's help about the mechanism about how it works.
Could someone help me?

thanks