Interesting problem we had the other day. Configuring a 2950T for ad-hoc use ... must be able to connect to any of the access switch stacks in the building. There are many subnets/vlans to work in so the configuration was simple .... all ports configured as 'access', all in vlan 1 (default) and bpdufilter enabled ... as bpduguard is enabled on all the access switch ports it's likely to connect to.
Connected one port back to the floors switch stack and one port to a laptop. Layer 2's up but there's no connectivity to the subnets default gateway. Checked and rechecked the configs, no surprises there. Running Ethereal on the laptop showed no responses to ARP's ... what you would see if the laptops switchport and the switchport going back to the switch stack were in different vlans.
Output from 'show vlan' omitted the port we'd connected back to the switch stack ... it just wasn't there. Move that patch to a different port on our 'ad-hoc' switch and it would 'disappear' from 'show vlan' results..
Grabbed another 2950T with a minor IOS version difference but same config and worked fine.
Bug???
BernieM