First of all, freelance editors and writers are often sent boilerplate contracts that include clauses requiring liability insurance--which might make sense if you are rebuilding a wing of a building, but rarely make sense for a writer and certainly not for an editor. My advice (gained from others' experiences): Cross out that clause and tell the client that it isn't relevant because (if true) you don’t have employees, work onsite, travel on behalf of the client, see clients in your home office, operate heavy equipment, endanger the general welfare, and so forth. (H/T to Ruth E. Thaler-Carter for the wording.) If you raise an objection based on common sense, the client is likely to tell you just to strike the clause. If they don't and the fee is low, it probably won't make financial sense to sign the contract, or at least I would not do so. See also what Will MacPheat reports, in the Comments section.
Meanwhile, here's the best round up of information I have come up with for if you DO find yourself in a situation where you need to buy the insurance. Media perils liability insurance (or publishers liability insurance) may provide you with protection for such traditional claims as copyright infringement; libel; defamation, plagiarism; invasion or privacy or publicity or infringement of privacy or publicity rights; misappropriation of trademark, title, or slogan; defamation; misappropriation of property rights; personal injury; contextual errors and omissions.
Media liability insurance is a specialized form of errors and omissions policy specifically designed for media clients who are at risk from liability claims brought by third parties.
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