Drupal 8 errors and their solutions

Error: (language.entity.en, language.entity.und, language.entity.zxx, language.mappings, language.negotiation, language.types) provided by language already exist in active configuration

Cause: I had enabled these modules in the GUI locally, then done some combination of pulling and pushing with Pantheon dev, and now I have an error.

Fix: delete yml files in config folder with those names, go to

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Error: a bunch of stuff telling you source is an undefined index after you enable language and translation and add a few languages

Cause: no path

Fix: admin/config/regional/language/detection/url

Tutorial that would have avoided this: https://docs.google.com/document/d/1XzurspMaw-Sf5SSMTpCUbVwWqp-12oWYhEn6DuN7Wss/edit#heading=h.pj1g6noocdom

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Error: 'Drupal\Core\Config\StorageException' with message 'Failed     [error] to create config directory sites/default/config'

Cause: config dir not writeable on Pantheon

Fix: switch to SFTP mode when saving config changes on Pantheon

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Error: 8.1.1 upgrade merge conflict that resolves badly

Fix: git pull git://github.com/pantheon-systems/drops-8.git [BRANCHNAME]

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Error: Translation won’t save, log shows Column not found: 1054 Unknown column 'content_translation_source'

Cause: missing columns in database

Fix: admin/config/regional/content-language, Save configuration

Source: https://www.drupal.org/node/2599228