Anerve cell is at rest. what would happen if something mistakenly triggered the simultaneous opening of many cl- ion channels?

I believe that the Chlorine ions would rush into the cell, bringing the cell closer to threshold.
The open chloride channels allow negatively charge chlorine into the cell, which inhibits the cell from firing. Chloride ions are negatively charged and also have a higher concentration outside the neurons in the extracellular space. When the chloride channels at the synapse open, the chloride ions diffuse into the cell, bringing their negative charge with them. The net effect is to move the membrane potential of the cell to be more negative.