On a company main switch (Cisco 3750) I have connected a simple Windows XP physical PC on a switchport (switchport mode access, switchport access vlan 30). NO monitor sessions exists on the 3750. Also, the PC has a single IP address on a single NIC. However, when I start a Wireshark promiscuous capture on the WinXP PC, several foreign conversations appear, either LAN-to-LAN or WAN-to-LAN (that is src IP and dst IP does not match the PC's IP address). This traffic is NOT broadcast (neither L2 broadcast nor L3 broadcast). The switch's configuration is a simple layer 3 and layer 2 configuration. Nothing fancy (ommited for cluttering the reasons)
I have checked the switch's MAC and ARP table and all seems normal: the PC's switchport shows only 1 MAC address and the other foreign MAC and IP addresses resolve to their normal ports (using "show ip arp" and "show mac-addr" etc.)
The whole capturing idea was initiated because we want to troubleshoot frequent application lags (according to other captures, many TCP retransmissions are the possible culprit).
EDIT after an IOS research it seems that there is a pretty big chance that my IOS version (12.2(25)SEB4) may have serious bugs about CAM tables and such. I will upgrade next week and revisit to update.