The problem comes when markets start falling and investors get the dreaded margin call. Using the same example if the shares fall to £80 on the first day the investors entire £10 has been wiped out, plus another £10 of debt he now owes. However, at the end of the day the broker will only call for an additional £2 to be put into the account. If at the end of the following day the shares fall further and the investor cuts their losses, they have to find money to repay their debts.