In Chapter 11 of The Chrysalids, what forbidden thing does Petra do?
The primary thing that Petra does that is forbidden is to communicate telepathically as a deviant Blasphemy. Other than that, in Chapter 11, Petra rides her pony into the forbidden forest area. Once there, she falls prey to danger and screams violently, sending thoughts of terror screeching outward. David, Rosalind and Michael are all compelled to run to the source of the telepathic cries.
Petra has again caused a serious situation that could lead to the telepaths being exposed. For one thing, Jerome Skinner has followed into the forest to see what the commotion was about. The three telepaths who converged to rescue Petra are able to divert Jerome's suspicions but they realize there is a growing threat around them. They decide that in future Petra's cries will be answered by David, then Rosalind and then by anyone within reach. Petra is to be rendered unconscious as soon as she is reached.
Uncle Axel advises that David has gotten careless and admonishes him to use greater care because people are starting to ask questions. As a result of Uncle Axel's cautions, the telepaths plan to be on guard and prepared for sudden escape if things go against them.