This concerns an open issue from before the public release, which I
had not understood before and believe to have understood now.

With this code *) added to the test

@@ -5946,6 +5949,15 @@ t_cache(unsigned chksum)
	test_bocdone(fbo, TRUST_ME(hash), 1);
	fellow_cache_obj_deref(fc, fco);

+	// === max out region alloc
+	fbo = fellow_busy_obj_alloc(fc, &fco, &priv2, 1234).r.ptr;
+	CHECK_OBJ_NOTNULL(fbo, FELLOW_BUSY_MAGIC);
+	for (u = 0; u < FCO_MAX_REGIONS; u++)
+		AN(fellow_busy_region_alloc(fbo, 1234, INT8_MAX));
+
+	test_bocdone(fbo, TRUST_ME(hash), 1);
+	fellow_cache_obj_delete(fc, fco, hash);
+
	// === alloc space, dont use

we tripped here

   assert(flivs->oob || u == obj_alive);

with u == 1 and obj_alive == 0.

So the offset of a region from a dead object was not taken by a
subsequent allocation, which is fine, why should it be?

*) Note: The added test code is not correct yet, as it does not
   register the regions with the segment list, so obj_delete leaks.