The Wrath of Janus

What went wrong? 

     (define janus (list 99
                  -->    3.2e+270
                         -99
                  -->    -2.4e+270
                         1234
                         4
                  -->    -8e+269
                         -4))
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
           +1e20
           -1e20
           99)
Add up the list in each direction yourself, remembering to only worry about the most significant 15 digits, ignoring the relatively small.