There is a growing tendency amongst people to forward emails and copy people in. This sometimes even happens between people involved in a financial transaction or even where there is a dispute. When it comes to communications between you and your lawyer letting anyone else see them is really not a good idea. Legal advice is meant for the client’s eyes only.
Sharing confidential information can sometimes lead to particularly serious problems where a matter is contentious, or becomes so. If the court thinks someone has revealed some of their confidential (or ‘privileged’) communications it may order that all which should have remained secret must be disclosed.