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+## Arrays implicitly concatenate in [[ ]]. Use a loop (or explicit * instead of @).
+### Problematic code:
+
+```sh
+ext=png
+allowedExt=(jpg bmp png)
+[[ "$ext" = "${allowedExt[@]}" ]] && echo "Extension is valid"
+```
+
+### Correct code:
+
+```sh
+ext=png
+allowedExt=(jpg bmp png)
+for value in "${allowedExt[@]}"
+do
+  [[ "$ext" = "$value" ]] && echo "Extension is valid"
+done
+```
+### Rationale:
+
+Array expansions in `[[ .. ]]` will implicitly concatenate into a single string, much like in assignments. The problematic code is equivalent to `[ "$ext" = "jpg bmp png" ]`. 
+
+Instead, use a `for` loop to iterate over values, and apply your condition to each.
+
+Alternatively, if you do want to concatenate all the values in the array into a single string for your test, use `"$*"` or `"${array[*]}"` to make this explicit.
+
+### Exceptions:
+
+None.
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