The error is caused by modifying a register on two or more different processes, with the first instance found on the line mentioned by the error message (line_num).
Sample Code:
module sample_multiple_instance(in1, in2, out1); input in1,in2; output out1;
reg samp_reg; assign out1 = samp_reg; always@(in1) begin samp_reg <= samp_reg-1; end always@(in2) begin samp_reg <= samp_reg+1;
end endmodule
ERROR - .../sample.v(13): net out1 is constantly driven from multiple places at instance sub_3, on port o[0].
The register that is causing the error is "samp_reg" which being modified on two processes, the first of which is found on line 13.