To "touch," "hit," "strike" a raw nerve - to upset someone by reminding him or her of a subject about which he or she has strong feelings.
1) Alex's mom recently died of lung cancer, so his sister hit a raw nerve by telling him that she had begun smoking.
2) Pam struck a raw nerve in Nick when she told him an anti-Semitic joke. Unbeknownst to Pam, Nick's grandmother died in the Holocaust.
3) Fat jokes touch a raw nerve in Samantha, who used to get made fun of in school for being overweight.