Screaming Penguin - Source Does Matter

Most computer users are friendly with ICMP on one way or another whether they realize it or not. ICMP is the Internet Control Message Protocol and is is one of the protocols in the TCP/IP family (Transmission Control Protocol Internet Protocol.)
The most common ICMP command many people see and use is the old standby, the Packet Internet Groper or 'ping'.

ICMP has many useful features and provides a great deal of information about netoworks. ICMP is very useful for internal TCPIP traffic and for knowledgeable system administrators or other automated network analysis tools.

The same benefits become drawbacks when you consider that malicious users can also use ICMP to find out a lot more than you want them to know about your network and to potentially cause damage.

ICMP features such as "Redirect", "Source Quench", "Time Exceeded" and more can all be used as potential attack vehicles (in one way or another.)

In fact there are 52 ICMP types and only 1 is recommended as "non-venemous." The "Destination Unreachable" type is allowable and beneficial for its participation in Path MTU (Maximum Transmission Unit) discovery.

The linked article by networkMagazine and the attached PDF by security professional Olif Arkin are more specific and detailed, see those resources for further info.
  ICMP Stands for Trouble: networkMagazine