excerpt from Fuegos link.
When clipping runners it is easy to tangle the ropes up, for example by clipping out of order, and thus increase drag rather than avoid it.

I watched a girl in redrocks make this error. She had the idea in her head that you HAVE TO alternate clips. By alternating clips she'd increased drag and tangled the ropes to the point that she couldn't move up no matter how much slack her belayer gave her. Daryl at Nomad's explained it to me once this way, "imagine each half as a single and keep that rope as straight as possible". So sometimes you might clip the same rope twice in a row if you had to in order to keep a straight run and not introduce drag into the system.