What went wrong, Alpine 3808 <-> Cisco 3560G. Cisco hangs.

We have a small flat (non routed) network.  Originally it looked something like this:

internet <-> router <-> firewall <-> ALPINE 3808 switch <-> all our PC & Unix machines

We installed a Cisco 3560G in the network so that all the UNIX boxes can run at Gig network amoung themselves.  
So now internally it is something like this:
  UNIX machines 1000 mb <-> Cisco <-> ALPINE <-> PC & Misc servers 100mb
The Cisco 3560G uplink plugs back into the ALPINE 3808

I hope I've given a good idea of what the network looks like.  Now the problem -
Someone plugged one of the ports of the Cisco 3560G into a port on the ALPINE, regular cat5 cable ... and the Cisco hung.  We had to (unplug it from the Alpine) and reboot it to get that segment of the network back up and running.

What did we do wrong?  What basic networking concept are we missing?