It's not clear from the documentation what types of values could be contained within an API response. For example, in the documentation for trends/weekly (https://dev.twitter.com/docs/api/1/get/trends/weekly) it's not clear what possible values for "events" might be. Or, in the documentation for geo/id/:place_id (https://dev.twitter.com/docs/api/1/get/geo/id/%3Aplace_id) it's not clear what all the possible values for "place_type" might be ("neighborhood" and "city" are shown in the example, but it would be nice to have a comprehensive list that is updated as new types of places are added).
Updates
The API long assumed familiarity with core Twitter concepts, nouns, verbs, and so on rather than the direct documentation of fields. We have a project to better rectify this that will eventually land on dev.twitter.com as a kind of "field guide" to identifying fields in the wild. In the meantime, you can observe the incremental progress at https://github.com/episod/twitter-api-fields-as-crowdsourced/wiki
