Once a woman who had never flown on an airplane before, decided to visit her son in California. Before buying the ticket she wanted to get a blessing from the Rebbe, which she did. Shortly after takeoff, one of the wings caught on fire and the pilot had to make an emergency landing. Afterwards, the woman, upset and furious, ran into the Rebbe's office yelling - "You knew what was going to happen to this flight! How could you have given me your blessing??!"
The Rebbe's response was: "Because of your merits, many Jews on this flight were saved. Is that bad?…"
(as related by R. Yoel Kahn)