In every version of Minecraft, third party (hacked) clients come along and manage to instantly lock multiplayer (i.e. cause SMP to freeze/go unresponsive). I assume this is a result of some kind of buffer overrun defect.
I have no proof that the behavior is actually caused by a buffer overrun, it's just what I call it when it happens.
Although managed languages such as Java and C# do not actually have buffer overruns, the specific virtual machines (JVM/CLR/etc) which actually run the code may.
Whatever it really is, something in a third party client is capable of generating a packet that can instantly choke the server, leaving it completely unresponsive, even to OS signals.
Analysis of the log usually gives the SMP staff clues and patterns of behavior to help mitigate the problem after the fact. Players using these techniques tend to announce they are about to crash the server, then do so. Banning the player and their IP helps reduce further occurrences.
The server does not typically generate a crash report for this situation. It just freezes, although the garbage collector appears to continue running.
The following is a log segment of it occurring today. The staff did not notice the problem for forty minutes (note the time stamp). In this example, the player (name redacted because I'm sure they love publicity) performed the same attack less than 24 hours prior. Their player name and IP was banned based on this pattern.