Seems like no matter how much we communicate the fact that an estimate is a mere guess, inevitably client’s (and rightly so) want to hold you accountable in the event circumstances lead to an increase in cost. Estimating is not a crystal ball activity, but there are tactics to be deployed to keep costs in check along the way. Pay attention to change orders to manage project sprawl and factor in a contingency (percentage over cost) to give you a little wiggle room to help mitigate some of the unexpected ‘inevitabilities’ that require time to address. Come in under budget to the estimate and everybody’s a hero.