The Wrath of Janus

What went wrong?

(define janus (list 99
             -->    3.2e+270
             -->    -2.4e+270
             -->    -8e+269
Look at the three indicated expressions in the list. These are the largest entries, ludicrously dwarfing everything else by a factor of around 10265. Notice that they cancel each other out.

Here's a smaller list that has the exact same problem:

(list 1
Add up the list in each direction yourself, remembering to only worry about the most significant 15 digits, ignoring the relatively small.