Conservative GC is quite vicious against the use of lazy lists:

You have code like this:

  function(make_infinite_lazy_list());

where function walks down the list:

  fun function(list)
  {
     while (list) {  // nil is false
        ...
        list = cdr(list);
     }
  }

problem is, the parent stack frame contains a spurious copy of the original return value from make_infinite_lazy_list, and so as function marches down the list, the discard prefix of the list is not becoming garbage, as expected.

This is the showstopper for conservative GC. Not the bogeyman of a stray machine integer suddenly looking exactly like a heap pointer.

Stick a conservative scanner under C, make yourself some lazy lists, and this problem will easily reproduce!