William R. Sauer's medical, drug and alcohol problems since childhood left him 'physically and mentally incapacitated.' He informed a hospital clerk he carried Blue Cross Health Insurance. He been covered from 1980 until March 1, 1984. His father, Robert T. Sauer, personally paid all of his son's Blue Cross premiums. William gave his address as P.O. Box 176, Chesterfield, Missouri. This post office box was owned by and is the principal address of the R.T. Sauer Agency. Robert T. Sauer is the president, sole shareholder, and director of the R.T. Sauer Agency which operates out of his home. A hospital employee, who later made a computer search for his Blue Cross certificate number, failed to transmit William's middle initial. They assumed that William (D) was William J. Sauer of Milwaukee, Wisconsin. Blue Cross mistakenly mailed sixty-six checks to William Sauer at P.O. Box 176, Chesterfield, Missouri. Thirty-three of the checks, totaling $10,108.36, were endorsed by William R. Sauer's signature or signature stamp; twenty-eight checks, totaling $3,874.72, were endorsed by William R. Sauer and the R.T. Sauer Agency; three checks, totaling $1,267.20, were endorsed only by the R.T. Sauer Agency; and two checks, totaling $6,773.01, were endorsed by William R. Sauer and Robert T. Sauer. Checks endorsed by the R.T. Sauer Agency went into the corporate account. Checks with Robert T. Sauer's personal signature went into his personal account. Blue Cross (P) sued to get the money back from Ds. P alleges that Ds were unjustly enriched by the retention of money paid to them inadvertently or by mistake. P sought a constructive trust upon those funds.