echo $(( 16 - 08 ))
echo $(( 16 - 8 ))
ShellCheck found an integer literal with a leading zero, but containing the digits 8 or 9.
This is invalid, as the integer will be interpreted as an octal value (e.g. 0777 == 0x1FF == 511).
To have the value parsed in base 10, either remove the leading zeros as in the example, or specify the radix explicitly:
The BASH manual "Shell Arithmetic" chapter
ShellCheck is a static analysis tool for shell scripts. This page is part of its documentation.