This precludes porting LilyPond to Guile 2 since LilyPond relies
extensively on the mark_smob functionality.

The symptom likely occurs when an object is collected and consequently
its free_smob function gets called.  This free_smob function essentially
is responsible for freeing all resources claimed by the smob, rendering
the underlying data moot.

If mark_smob continues to get called, its interpretation of the now dead
data is not likely to lead to happy results.

Debugging is tricky since the gc phases tend to occur asynchronously.
If I start the accompanying program with
./test 2 20000 40
I tend to get a core dump perhaps every fourth call.  Running repeatedly
in a debugger does not produce core dumps (perhaps one needs to reload
the debugger for each try?), so one needs to do

ulimit -c unlimited
./test 2 20000 40

and, after the problem triggers

gdb ./test core

for post-mortem debugging.  A traceback looks like
(gdb) bt
#0  0x00000000 in ?? ()
#1  0x0804aee0 in ?? ()
#2  0xb761b2da in ?? () from /usr/lib/libguile-2.0.so.22
#3  0xb72751f8 in GC_mark_from () from /usr/lib/i386-linux-gnu/libgc.so.1
#4  0xb72766ca in GC_mark_some () from /usr/lib/i386-linux-gnu/libgc.so.1
#5  0xb726cd16 in GC_stopped_mark () from /usr/lib/i386-linux-gnu/libgc.so.1
#6  0xb726d730 in GC_try_to_collect_inner ()
   from /usr/lib/i386-linux-gnu/libgc.so.1
#7  0xb726e12a in GC_collect_or_expand ()
   from /usr/lib/i386-linux-gnu/libgc.so.1
#8  0xb726e2b1 in GC_allocobj () from /usr/lib/i386-linux-gnu/libgc.so.1
#9  0xb72731a4 in GC_generic_malloc_inner ()
   from /usr/lib/i386-linux-gnu/libgc.so.1
#10 0xb72732be in GC_generic_malloc () from /usr/lib/i386-linux-gnu/libgc.so.1
#11 0xb761b81e in scm_i_new_smob () from /usr/lib/libguile-2.0.so.22
#12 0x0804985a in std::vector<void (*)(), std::allocator<void (*)()> >::_M_insert_aux(__gnu_cxx::__normal_iterator<void (**)(), std::vector<void (*)(), std::allocator<void (*)()> > >, void (* const&)()) ()
#13 0x0804a6fc in __gnu_cxx::new_allocator<void (*)()>::allocate(unsigned int, void const*) ()
#14 0x0804a03d in void std::_Destroy<void (**)(), void (*)()>(void (**)(), void (**)(), std::allocator<void (*)()>&) ()
#15 0x08049a73 in void std::_Destroy<Family**, Family*>(Family**, Family**, std::allocator<Family*>&) ()
#16 0x0804934d in scm_puts ()
#17 0x0804938f in Scm_init::Scm_init(void (*)()) ()
#18 0x0804938f in Scm_init::Scm_init(void (*)()) ()
#19 0x0804938f in Scm_init::Scm_init(void (*)()) ()
#20 0x0804938f in Scm_init::Scm_init(void (*)()) ()
#21 0x0804938f in Scm_init::Scm_init(void (*)()) ()
#22 0x0804938f in Scm_init::Scm_init(void (*)()) ()
#23 0x0804938f in Scm_init::Scm_init(void (*)()) ()
#24 0x0804938f in Scm_init::Scm_init(void (*)()) ()
#25 0x0804938f in Scm_init::Scm_init(void (*)()) ()
#26 0x0804938f in Scm_init::Scm_init(void (*)()) ()
#27 0x0804938f in Scm_init::Scm_init(void (*)()) ()
#28 0x0804938f in Scm_init::Scm_init(void (*)()) ()
#29 0x0804938f in Scm_init::Scm_init(void (*)()) ()
#30 0x080495e7 in std::vector<Family*, std::allocator<Family*> >::operator[](unsigned int) ()
#31 0xb75b7dfd in ?? () from /usr/lib/libguile-2.0.so.22
#32 0xb76418e7 in ?? () from /usr/lib/libguile-2.0.so.22
#33 0xb761afb9 in ?? () from /usr/lib/libguile-2.0.so.22
 #34 0xb7659f20 in ?? () from /usr/lib/libguile-2.0.so.22
#35 0xb765a539 in ?? () from /usr/lib/libguile-2.0.so.22
#36 0xb75c24f3 in scm_call_4 () from /usr/lib/libguile-2.0.so.22
#37 0xb7641acf in scm_catch_with_pre_unwind_handler ()
   from /usr/lib/libguile-2.0.so.22
#38 0xb7641bd4 in scm_c_catch () from /usr/lib/libguile-2.0.so.22
#39 0xb75b85d1 in ?? () from /usr/lib/libguile-2.0.so.22
#40 0xb75b86d3 in scm_c_with_continuation_barrier ()
   from /usr/lib/libguile-2.0.so.22
#41 0xb763ef7e in ?? () from /usr/lib/libguile-2.0.so.22
#42 0xb72782c1 in GC_call_with_stack_base ()
   from /usr/lib/i386-linux-gnu/libgc.so.1
#43 0xb763f3e6 in scm_with_guile () from /usr/lib/libguile-2.0.so.22
#44 0x080496c5 in void __gnu_cxx::__alloc_traits<std::allocator<void (*)()> >::construct<void (*)()>(std::allocator<void (*)()>&, void (**)(), void (* const&)()) ()
#45 0xb72cfa83 in __libc_start_main (main=0x4, argc=134517216, argv=0x0, 
    init=0x8049201 <main+16>, 
    fini=0x804967e <std::_Vector_base<void (*)(), std::allocator<void (*)()> >::~_Vector_base()+40>, rtld_fini=0x4, stack_end=0xbfca7aa4) at libc-start.c:287
#46 0xb7750000 in ?? () from /lib/ld-linux.so.2
Backtrace stopped: previous frame inner to this frame (corrupt stack?)

The frames listed as

#17 0x0804938f in Scm_init::Scm_init(void (*)()) ()
#18 ...

appear to be a fluke in address/symbol correlation and do not seem to
have an actual relation to the listed function when looking at the code.

I append all the requisite source files producing the test binary when
running "make".  I can send my binary (when desired) which was compiled
using 2.0.11 on a i586 platform.  However, I would expect the problem to
reproduce in some kind of manner on other systems.

The compiler was gcc version 4.9.1 (Ubuntu 4.9.1-16ubuntu6), Target:
i686-linux-gnu.

The headers are somewhat cooked-down variants (to remove dependencies on
other parts of LilyPond) of the Smob organizing classes used in
LilyPond.  If one wants to compile for Guile v1 for comparison, one
probably needs

typedef void * scm_t_func;

or so somewhere.