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+## This action ignores everything before the -o. Use \\( \\) to group.
+
+### Problematic code:
+
+    find . -name '*.avi' -o -name '*.mkv' -exec cp {} /media \;
+
+### Correct code:
+
+    find . \( -name '*.avi' -o -name '*.mkv' \) -exec cp {} /media \;
+
+### Rationale:
+
+In `find`, two predicates with no operator between them is considered a logical, short-circuiting AND (as if using `-a`). E.g., `-name '*.mkv' -exec ..` is the same as `-name '*.mkv' -a -exec ..`.
+
+`-a` has higher precedence than `-o`, so `-name '*.avi' -o -name '*.mkv' -a -exec ..` is equivalent to `-name '*.avi' -o \( -name '*.mkv' -a -exec .. \)`. 
+
+In other words, the problematic code means "if name matches `*.avi`, do nothing. Otherwise, if it matches `*.mkv`, execute a command.". 
+
+In the correct code, we use `\( \)` to group to get the evaluation order we want. The correct code means "if name matches `*.avi` or `*.mkv`, then execute a command", which was what was intended.
+
+### Contraindications
+
+If you're aware of this, you can either ignore this error or group to make it explicit. For example, to decompress all gz files except tar.gz, you can use:
+
+    find . -name '*.tar.gz' -o \( -name '*.gz' -exec gzip -d {} + \)