Interesting, but given that the code in question computes (expt 2 488) as the square of (expt 2 244), is there actually any way for that bug to cause the observed symptoms?

(2^244 is being shown as zero. 2^488 is being shown as non-zero; presumably the right value though I haven't actually checked. If 2^244 is actually being computed as zero, then unless what's happening somehow depends on the depth of the stack -- surely not possible if this bug is the cause -- 2^488 should be computed as zero squared, which is zero.)