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+## You probably wanted && here
+
+### Problematic code:
+
+    if [[ $1 != foo || $1 != bar ]]
+    then 
+      echo "$1 is not foo or bar"
+    fi
+
+### Correct code:
+
+    if [[ $1 != foo && $1 != bar ]]
+    then 
+      echo "$1 is not foo or bar"
+    fi    
+
+### Rationale:
+
+This is not a bash issue, but a simple, common logical mistake applicable to all languages.
+
+`[[ $1 != foo || $1 != bar ]]` is always true:
+
+* If `$1 = foo` then `$1 != bar` is true, so the statement is true.
+* If `$1 = bar` then `$1 != foo` is true, so the statement is true.
+* If `$1 = cow` then `$1 != foo` is true, so the statement is true.
+
+`[[ $1 != foo && $1 != bar ]]` matches when `$1` is not `foo` and not `bar`:
+
+* If `$1 = foo`, then `$1 != foo` is false, so the statement is false.
+* If `$1 = bar`, then `$1 != bar` is false, so the statement is false.
+* If `$1 = cow`, then both `$1 != foo` and `$1  != bar` is true, so the statement is true.
+
+This statement is identical to `! [[ $1 = foo || $1 = bar ]]`, which also works correctly.
+
+### Contraindications
+
+None.
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