sh (at least between 2.0.2 and 3.0 - 1.6.1 is OK) doesn't seem to like 
leading zeros on 8 and 9, but is OK with all other numbers:

# echo $((05+1))
6
# echo $((06+1))
7
# echo $((07+1))
8
# echo $((08+1))
arithmetic expression: syntax error: "08+1"
# echo $((09+1))
arithmetic expression: syntax error: "09+1"
# echo $((10+1))
11

This is down to the following in arity_lex.l rev 1.11 and later:
0x[0-9a-fA-F]+  { yylval = strtol(yytext, 0, 16); return(ARITH_NUM); }
0[0-7]*         { yylval = strtol(yytext, 0, 8); return(ARITH_NUM); }
[1-9][0-9]*     { yylval = strtol(yytext, 0, 10); return(ARITH_NUM); }

I see from the 1.11 commit this is to recognise octal and hex, but must a 
leading zero always indicate octal?

-- 
Stephen