account A
account B
commodity GBP

2012-03-25 GBP
    A            10.00 GBP
    B

2012-03-26 EUR
    A            20.00 EUR
    B

$ hledger -f- check

#test bal --pedantic -> 1
#__ERROR__
#While parsing file "$FILE", line 10:
#While parsing posting:
#  A            20.00 EUR
#               ^^^^^^^^^
#Error: Unknown commodity 'EUR'
#end test
#
#