Due to outstanding driver issues with hardware encryption acceleration, users of Intel WiFi Link 4965, 5100, 5150, 5300, and 5350 wireless cards are advised to disable hardware accelerated encryption via module parameters. Failure to do so may result in the inability to connect to WEP (Wired Equivalent Privacy) protected wireless networks after connecting to WPA (WiFi Protected Access) protected wireless networks.
To do so, add the following options to /etc/modprobe.conf:
alias wlan0 iwlagn
options iwlagn swcrypto50=1 swcrypto=1
(where wlan0 is the default interface name of the first Intel WiFi Link device; replace or amend as appropriate for local hardware configuration)