[TYPO3] Strange record labels after itemsProcFunc in TCA

[Simon Tuck]

Hi List,
Quick TCA question: I'm using itemsProcFunc to change the foreign table 
field for some of my tables. These same foreign table fields are also in the 
record labels in the list view (i.e. using label_alt & label_alt_force). 
This all works fine, but I'm curious: sometimes the labels show up as 
intended, for example: 123456789, 12, Test; other labels however show up as 
123456789, [12!], [567623767!]. Could somebody explain the meaning of the 
brackets and the exclamation? Does this point towards a potential problem or 
is there an error in my TCA?
Cheers,
Simon