Ok, the poparena has already pointed this out, but since he hasn't posted it here, I figured I should.
How about in #6. The Capture when they left Rachel alone to pretend to sleep in front of Controller Jake? Sure, some might argue that this was a Rachel idea, but they all agreed on it for it to have to go forward. Seriously, if the Yeerk had been cautious enough, he would have killed her! What's four more morph-capable bodies compared to five, anyway? They were relying too much on the Yeerk's desire to have more morphing hosts than anything, which could have backfired horribly in their faces and left them less equipped to deal with a renegade Yeerk on the loose.
Discuss.