Check that the string compiles.
If the source phrase is broken, we have to die at run time. If the target phrase is broken, we do not get the translation.
Conceivably there could also be more subtle problems it could cause.
There was a problem compiling the string.
The string is replaced with a comment that details the problem, typically including an escaped verison of the problematic string: [comment,Bracket Notation Error: DETAILS_GO_HERE]