Small or Multi-billion dollar case, legal malpractice cases all rest upon the same three bases:  deviation from accepted practice, proximate cause and damage.  As an example, here is a smallish case from the Madison Record:

"A legal malpractice claim filed by Cydney Hollaway against Fairview Heights attorney Thomas C. Rich alleges she did not receive an adequate award for injuries in a workers’ compensation claim.

Hollaway, represented by Patricia A. Zimmer of Ripplinger & Zimmer in Belleville, is seeking in excess of $50,000 in damages plus costs of the suit.

According to the suit filed Jan. 22 in St. Clair County Circuit Court, Rich allegedly presented Holloway’s claim to an arbitrator as one for wage differential instead of one for permanent total disability.

Holloway was injured while working for Four Truckers, Inc. on Feb. 8, 2001, the suit claims. Holloway contracted with Rich on Feb. 15, 2001, whereby Rich would represent Holloway in a work comp claim.

"On December 13, 2005, defendant presented plaintiff’s claim at a hearing before Illinois Workers Compensation Commission Arbitrator John Dibble," the complaint states.

"On January 23, 2006, Arbitrator Dibble issued his decision, which awarded plaintiff compensation in the form of a wage differential."

Holloway claims she had a viable claim for permanent total disability and would have been successful in obtaining an award, "but for defendant’s negligence."