Looks like the problem occurs in C_rereclaim:

On my box at work, it doesn't fail:

peter@chewbacca% LD_LIBRARY_PATH=.. ../csi -n -include-path .. -s -:g -:hi120K lolevel-tests.scm
[GC] (old) fromspace:   start=0x000000000070e7c0,       limit=0x000000000071d7c0
[GC] (old) tospace:     start=0x000000000071d7d0,       limit=0x000000000072c7d0
[GC] resized heap to 2220032 bytes
[GC] (new) fromspace:   start=0x00007f19f9657010,       limit=0x00007f19f9766010
[GC] (new) tospace:     start=0x00007f19f8581010,       limit=0x00007f19f8690010
reclaim p[1] = 0x7fff19b28d70, p[0] = 0x7f19f96b1f28
reclaim p[1] = 0x7fff19b122b0, p[0] = 0x7fff19b80e60
[GC] level  1   gcs(minor)  24  gcs(major)  1
[GC] stack      0x00007fff19a84f20      0x0000000019b0fd00      0x00007fff19b84f20
[GC]  from      0x00007f19f8581010      0x00007f19f85cb418      0x00007f19f8690010      0x000000000004a408
[GC]    to      0x00007f19f9657010      0x00007f19f9657010      0x00007f19f9766010 
[GC] 0 locatives (from 32)
reclaim p[1] = 0x7fff19af3080, p[0] = 0x7f19f85afa08
[GC] level  1   gcs(minor)  1   gcs(major)  2
[GC] stack      0x00007fff19a84f20      0x0000000019af08f0      0x00007fff19b84f20
[GC]  from      0x00007f19f9657010      0x00007f19f96a1888      0x00007f19f9766010      0x000000000004a878
[GC]    to      0x00007f19f8581010      0x00007f19f8581010      0x00007f19f8690010 
[GC] 0 locatives (from 32)
[GC] level  1   gcs(minor)  4   gcs(major)  3
[GC] stack      0x00007fff19a84f20      0x0000000019a9c5f0      0x00007fff19b84f20
[GC]  from      0x00007f19f8581010      0x00007f19f85cbe28      0x00007f19f8690010      0x000000000004ae18
[GC]    to      0x00007f19f9657010      0x00007f19f9657010      0x00007f19f9766010 
[GC] 0 locatives (from 32)


Until you force a reallocation by tweaking the heap size:

peter@chewbacca% LD_LIBRARY_PATH=.. ../csi -n -include-path .. -s -:g -:hi420K lolevel-tests.scm
[GC] (old) fromspace:   start=0x00007fad210b0010,       limit=0x00007fad210e4810
[GC] (old) tospace:     start=0x00007fad2107b010,       limit=0x00007fad210af810
[GC] resized heap to 2527232 bytes
[GC] (new) fromspace:   start=0x00007fad20f46010,       limit=0x00007fad2107a810
[GC] (new) tospace:     start=0x00007fad1feda010,       limit=0x00007fad2000e810
reclaim p[1] = 0x7fff45818500, p[0] = 0x7fad20fa0ec0
reclaim p[1] = 0x7fff45801a40, p[0] = 0x7fff458705f0
[GC] (old) fromspace:   start=0x00007fad20f46010,       limit=0x00007fad2107a810
[GC] (old) tospace:     start=0x00007fad1feda010,       limit=0x00007fad2000e810
RERECLAIM p[1] = 0x7fad1feda010, p[0] = 0x7fad20fa0ec0
RERECLAIM p[1] = 0x7fad1feda028, p[0] = 0x7fff458705f0
[GC] resized heap to 1263616 bytes
[GC] (new) fromspace:   start=0x00007fad1fc3c010,       limit=0x00007fad1fcd6410
[GC] (new) tospace:     start=0x0000000000754f90,       limit=0x00000000007ef390
[GC] level  1   gcs(minor)  24  gcs(major)  1
[GC] stack      0x00007fff457746b0      0x00000000457ff490      0x00007fff458746b0
[GC]  from      0x00007fad1fc3c010      0x00007fad1fc86418      0x00007fad1fcd6410      0x000000000004a408
[GC]    to      0x0000000000754f90      0x0000000000754f90      0x00000000007ef390 
[GC] 0 locatives (from 32)
reclaim p[1] = 0x7fff457e2810, p[0] = 0x7fad1fc6aa08
[GC] level  1   gcs(minor)  1   gcs(major)  2
[GC] stack      0x00007fff457746b0      0x00000000457e0080      0x00007fff458746b0
[GC]  from      0x0000000000754f90      0x00000000007a00f0      0x00000000007ef390      0x000000000004b160
[GC]    to      0x00007fad1fc3c010      0x00007fad1fc3c010      0x00007fad1fcd6410 
[GC] 0 locatives (from 32)

Error: unbound variable: foo

        Call history:

        <syntax>          #<invalid forwarded object>
        <syntax>          #<invalid forwarded object>
        <syntax>          (##core#quote #<invalid forwarded object>)
        <syntax>          #<invalid forwarded object>
        <eval>    (mutate-procedure! #<invalid forwarded object> (lambda (#<invalid forwarded object>) (lambda #<inval......
        <syntax>          (assert (not (eq? foo new-foo)))
        <syntax>          (##core#let ((tmp139 (not (eq? foo new-foo)))) (##core#if (##core#check tmp139) tmp139 (##sys#error ...
        <syntax>          (##core#begin (##core#if (##core#check tmp139) tmp139 (##sys#error "assertion failed" (##core#quote ...
        <syntax>          (##core#if (##core#check tmp139) tmp139 (##sys#error "assertion failed" (##core#quote (not (eq? foo ......
        <syntax>          (##core#check tmp139)
        <syntax>          (##sys#error "assertion failed" (##core#quote (not (eq? foo new-foo))))
        <syntax>          (##core#quote (not (eq? foo new-foo)))
        <syntax>          (not (eq? foo new-foo))
        <syntax>          (eq? foo new-foo)
        <eval>    (not (eq? foo new-foo))
        <eval>    (eq? foo new-foo)     <--