One drop while tracing - icmp unreachable & traceroute

Have you ever notice the * * while you are tracing some destination ?

Cochran#traceroute 192.168.16.1 probe 4

Type escape sequence to abort.
Tracing the route to 192.168.16.1

  1 172.20.15.5 4 msec 4 msec 0 msec 0 msec
  2 172.20.15.2 4 msec *  0 msec * 
Cochran#

When you ping no drops at all
Cochran#ping 192.168.16.1 repeat 4

Type escape sequence to abort.
Sending 4, 100-byte ICMP Echos to 192.168.16.1, timeout is 2 seconds:
!!!!
Success rate is 100 percent (4/4), round-trip min/avg/max = 4/5/8 ms
This behaviour due to ICMP unreachable rate limit configuration , only the last hop needs to generate icmp-unreachble others normally return the reply via ttl expired ( remember the way traceroute works ) 
Lindbergh#show ip int brief
Interface                  IP-Address      OK? Method Status                Protocol
FastEthernet0/0            unassigned      YES NVRAM  administratively down down    
Serial0/0                  172.20.15.2     YES NVRAM  up                    up      
FastEthernet0/1            192.168.16.1    YES NVRAM  up                    up      


Lindbergh#show ip icmp rate-limit 

                           DF bit unreachables       All other unreachables   
Interval (millisecond)     500                       500                      

Interface                  # DF bit unreachables     # All other unreachables 
---------                  ---------------------     ------------------------ 
Serial0/0                  0                         16                       
FastEthernet0/1            0                         0                        

Greatest number of unreachables on Serial0/0
check this information for more detail.
http://www.cisco.com/en/US/docs/ios/12_4t/12_4t2/hticmpun.html#wp1053877

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