Britney Spears' guest spot on C-B-S's How I Met Your Mother has sent another potential guest star, Alicia Silverstone, running from the show like her pants were on fire. The Clueless star was reportedly set to guest on the same episode, but pulled out when she learned of Britney's appearance.It's not that she was afraid that some of Brit's craziness would rub off on her -- she reportedly felt the atmosphere on the set would be too chaotic. Scrubs star Sarah Chalke will fill Silverstone's shoes. The episode airs March 24th.
