It looks like something's gone haywire. 17 octal ought to be F hexadecimal, not 11. What's wrong?
In short, always set obase before ibase , or you're in for a surprise.
Similarly, suppose you'd set ibase to 16, and want to set it back to 10. You need to type:
Another thing to look out for is typing a hex digit when ibase is set to something other than 16. bc isn't smart enough to reject the input, and gives you back garbage.