There is no single answer to this, but it’s wise to talk to your adviser and discuss any contractual liabilities you may face. Beware signing standard contracts without checking whether they increase your insurance exposure. For example, many US contracts will include a 'hold harmless' clause, which would prevent you or your insurers from recovering against a negligent party.
Often, the best form of protection is not insurance, but making sure you have strong terms of business that limit your possible exposure. Where you do make a mistake which affects a client, the best policies will intervene quickly to minimise the loss and help you retain the client.