If you use Twitter, you too are a public figure. And one egregious tweet could blow up your life.

On Tuesday, ABC DIS, +0.38%  cancelled the wildly successful reboot of “Roseanne” after Roseanne Barr wrote a racist tweet about Valerie Jarrett, a former adviser in Barack Obama’s White House. Barr apologized, but the damage was done. Several members of her cast and ABC executives denounced her.