Parallel Resistor Calculation -- Higher Equivalent Resistance
Mathematically put:
$$R_1 || R_2=\frac{R_1 R_2}{R_1 + R_2}$$
With your values:
$$1000=\frac{100 R_2}{100 + R_2}$$
Rearranging:$$
100000+1000R_2=100R_2$$
Or 
$$9R_2=-1000$$
So \$R_2=-111.11\Omega\$. Negative resistance.Physical? Not for a passive component such as resistor.