Variables in po files are of the form ${VARIABLE}; the spellchecker is able to catch the following common mistakes:

The variable name should not be translated in the msgstr and braces should not be changed. The variable name is replaced with something more useful at run-time; if the translator manipulates it, the results will be rather ugly. When the original msgid contains multiple occurrences of a particular variable, sometimes translators translate the string as to use the variable just once; the script will report such strings as wrong and it is up to the translator to check if it is a mistake or not (this is the main reason for calling them “suspect variables”).