AutoPkg needs to load all recipes in the search paths on every run so it can locate recipes and their parents (which may be spread over different repositories).
Because of this, AutoPkg may fail if a single recipe has malformed property list syntax. To locate the one broken property list among many, you can use the following command:
$ find ~/Library/AutoPkg -name "*.recipe" -exec plutil -lint {} \; | grep -v "OK$"
This uses find to find all files with the .recipe file extension in the AutoPkg folder and executes plutil -lint filename, then it uses grep to show only lines not (-v) ending with ‘OK’.