"The number one rule for outside law firms is these three words: Don't blindside me." 

Perhaps, through no one's fault, aspects of an engagement change — maybe there are many more documents to assess, a new priority arises, or a key player leaves the business. If the lawyer tells the client that things have changed, the client may not be delighted. But it is what it is. If clients the world over have one unifying characteristic, it is that they hate being blindsided.

It may be awkward to tell the client that the project has drifted and needs correction. But imagine the alternative, having to explain later that the project has drifted and you didn't spot it.