Assume out bit looks like this (0101011)Electronic errors. in other words , if the number of 1s in the frame is even and the flag is showing that correctly. Yet everything is looking great. but if receiver receives the frame and it will look like this (1010110), As you can see , The no. of 1s is still even , but the sequence if those bits is wrong. Parity check will still consider this frame as valid while in reality it is not because the order is of bits is wrong.