When you say very large numbers, you mean larger than 10e66, right?

This looks like a feature, not a bug.  That is, somebody has added a test for numbers so large no English words exist for them. You made me curious, what did older SBCL print for these numbers without names?  (I don't have any older version to test with here)

* (format t "~r" (expt 10 65))
one hundred vigintillion

* (format t "~r" (expt 10 66))

debugger invoked on a SIMPLE-ERROR in thread
#<THREAD "main thread" RUNNING {1002AEADC3}>:
  number too large to print in English: 1,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000

Stig Hemmer