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docs/common-issues.md

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+# Common Issues
+
+This page lists common issues you may encounter with PlaceholderAPI and how you can solve them.
+
+If you have more questions, feel free to join the [Discord Server](https://discord.gg/helpchat).
+
+## `java.lang.NoClassDefFoundError: com/google/gson/Gson`
+
+If you encounter an issue such as
+
+```
+org.bukkit.plugin.InvalidPluginException: java.lang.NoClassDefFoundError: com/google/gson/Gson
+```
+
+does it mean that the Server you're using PlaceholderAPI on does not have Gson included.  
+This is often the case for servers running 1.8 or older. To fix this, make sure to use at least 1.8.8 as that version does include the required dependency.
+
+## Expansions won't work
+
+If one or multiple expansions don't work, make sure you checked the following:
+
+- You executed `/papi reload` after downloading the expansion(s).
+- Any required plugin is installed and enabled.
+- The expansion is valid (See next sections).
+
+## `Failed to load expansion class <expansion> ...`
+
+### `- One of its properties is null which is not allowed`
+
+When this error appears does it mean that either `getAuthor()`, `getIdentifier()` or `getVersion()` in the expansion return `null` which is not allowed.  
+In such a case, contact the developer of the expansion and inform them about this issue and that it should be fixed.
+
+### `(Is a dependency missing?)`
+
+This error is given whenever the expansion cannot be loaded, which often happens due to a missing dependency (required plugin) or because creating an expansion instance failed.
+
+The only thing you can do is to provide the full error so that we can check if the issue is caused by PlaceholderAPI (More unlikely) or by the expansion.