Most modern operating systems support some form of network offloading, where some network processing happens on the NIC instead of the CPU. Normally this is a great thing. It can free up resources on the rest of the system and let it handle more connections. If you're trying to capture traffic it can result in false errors and strange or even missing traffic.
On systems that support checksum offloading, IP, TCP, and UDP checksums are calculated on the NIC just before they're transmitted on the wire. In Wireshark these show up as outgoing packets marked black with red Text and the note [incorrect, should be xxxx (maybe caused by "TCP checksum offload"?)].
Wireshark captures packets before they are sent to the network adapter. It won't see the correct checksum because it has not been calculated yet. Even worse, most OSes don't bother initialize this data so you're probably seeing little chunks of memory that you shouldn't.
New installations of Wireshark 1.2 and above disable IP, TCP, and UDP checksum validation by default. You can disable checksum validation in each of those dissectors by hand if needed.
If you are experiencing network problems and while trying to figure it out with Wireshark you found these checksum errors, you may have a network card with TCP checksum offload enabled and for some reason the packet is not being fixed by the adapter (NAT, bridge or route redirection is sending the packet to another interface). In this case, you may want to check and disable checksum offload for the adapter, if possible.
Checksum offloading can be enabled and disabled with the ethtool command.
ethtool --show-offload ethX
ethtool --offload ethX rx off tx off
Or, with some 3Com cards (see 3c59x vortex docs):
rmmod 3c59x ; modprobe 3c59x hw_checksums=0
In Windows, go to Control Panel->Network and Internet Connections->Network Connections, right click the connection to change and choose 'Properties'. Press the 'Configure...' button, choose the 'Advanced' tab to see or modify the "Offload Transmit TCP Checksum" and "Offload Receive TCP Checksum" values.
Some cards can reassemble traffic. This will manifest itself in Wireshark as packets that are larger than expected, such as a 2900-byte packet on a network with a 1500-byte MTU. You can check and change offloading behavior on Linux and Windows using the methods described in the previous section.
This article has a nice explanation on what to do.
Chimney offloading lets the NIC handle processing for established TCP connections. On Windows offloaded connections bypass WinPcap, which means that you won't capture TCP conversations.
netsh int ip set chimney disabled