Add another required case for using $ in an arithmetic expression, one that too many people tend to forget about.

SC2004.md

@@ -25,3 +25,15 @@ $ echo $((a * 5))     # evaluates as (1+1)*5
 ```
 
 The `$` is unavoidable for special variables like `$1` vs `1`, `$#` vs `#`. It's also required when adding modifiers to parameters expansions, like `${#var}` or `${var%-}`. ShellCheck does not warn about these cases.
+
+The `$` is also required (and not warned about) when you need to specify the *base* for a variable value:
+
+```sh
+$ a=09
+$ echo $((a + 1))          # leading zero forces octal interpretation
+bash: 09: value too great for base (error token is "09")
+$ echo $((10#a + 1))
+bash: 10#a: value too great for base (error token is "10#a")
+$ echo $((10#$a + 1))
+10
+```