From here: http://tools.ietf.org/html/rfc7049#appendix-A
There is this example:
0.00006103515625 -> 0xf90400
It turns out the left hand side is 2^-14.
Using libcbor 0.3.1 on XCode simulator (little-endian), cbor_serialize_alloc fails after cbor_build_float2 for the provided number.
Actually it appears to get into an endless loop until aborted by an illegal malloc request.
Here's a function that demonstrates the issue:

• (void) testTwoToMinus14
  {
  cbor_item_t* pItem = cbor_build_float2(0.00006103515625);
  unsigned char* buffer;
  size_t buffer_size, length = cbor_serialize_alloc(pItem, &buffer, &buffer_size);
  XCTAssert(length);
  }

Attached is an image showing my test program and the results.