Walking along a busy street in a safe neighborhood, your squadron mate is suddenly stabbed by a passerby. Unless you have trained, or have expected such a threat, this out-of-the-blue action leaves you suddenly in shock and disbelief – for just a moment, you are stunned. You are inactive. (It's the same theory behind those "flash and stun grenades" effective against hardened militants)
A Perfect example of disruption of the OODA loop.