I run an app on my Android smartphone that runs a WEP-encrypted ad-hoc network (so my laptop can get wireless internet through my phone's data plan). I can connect to this ad-hoc network without problem in Windows on this same computer, and it also worked when I used to run Ubuntu 10.10 on this computer.
I recently installed Ubuntu 12.04 (clean install, not upgrade), and now when I try to connect to the ad-hoc network through Ubuntu, it just keeps asking me for the network's passphrase and never connects. Moreover, once I've attempted to connect to this ad-hoc network, I cannot connect to any other wireless network until I reboot.
The computer in question is a Lenovo Thinkpad T410 with a Realtek RTL8191SEvB Wireless LAN Controller card.