Welcome to the wonderful world of Spanning Tree.
Everybody that has ever managed a network of reasonable size has these horror stories about STP. Everybody that has done support for a networking vendor has seen or heard first-hand stories about STP meltdowns.
I think it only takes one user in your organization that brings a cheap switch to the office that bridges, but does not do STP (properly), and your network can melt down. (E.g. it floods frames, but not BPDUs).
There are new technologies to attack this problem. No more STP meltdowns. And as a bonus, you can use multiple parallel links without creating loops.
TRILL - http://en.wikipedia.org/wiki/TRILL_%28computing%29
IEEE 802.1aq (aka Shortest Path Bridging) - http://en.wikipedia.org/wiki/IEEE_802.1aq
Two technologies that are very similar.
All switches talk a special new protocol, which resembles the IS-IS routing protocol.
This allows them to learn the topology of the network. And the location of all MAC addresses. Just like L1 routing with host-routes in IS-IS, but now at layer 2.
TRILL encapsulates frames between switches with a new header. This header has a TTL-field, which will suppress loops. IEEE802.1aq uses RPF (reverse path forwarding lookups) to drop looped packets. Cisco at the moment has its own flavor of TRILL, called FastPath. The future (and the market) will decide which of these 2 new protocols will win in the end.
As I am a big fan of the IS-IS routing protocol, I enjoy seeing the technology being used at layer-2. I'm curious to see how these protocols will develop.