Currently the config value for which files to preprocess are saved in a closure when a preprocessor is created. Which means that if a pattern is removed or added after the preprocessor is created, the new pattern is missed because the patterns were saved in a closure.

This is an issue when you're creating a plugin and you want to add a file that you also want to preprocess. That is, in your framework factory function, you add an entry to config.files and add an entry to config.preprocessors for that file. Because of this bug, the preprocessor is never run for the added file.