Nynaeve bests Moghedien in a duel in The Shadow Rising and subsequently shields her from the True Source, in addition to trapping her in Air (and tying the weave). However, Moghedien disappears literally seconds later while Nynaeve was momentarily distracted by balefire.
The shield of Spirit slid between Moghedien and the Source; the halo surrounding her winked out.
With no need to defend herself, it took only a little effort for Nynaeve to weave Air around the fleeing woman. The Forsaken froze in midstride. Hurriedly Nynaeve tied her weaving.
Suddenly she stared in disbelief. Moghedien was gone! The balefire had not come nearer than ten feet from where she had stood, but she was not there any longer. It was impossible. She had been shielded.
The Shadow Rising, Chapter 54, Into the Palace (TOR Fantasy page 910, emphasis mine)
Nynaeve was known to be exceptionally strong in the Power; certainly her weaving should have been strong enough to hold her for a mere few more seconds. So how did Moghedien escape?
I'm quite certain that this was mentioned somewhere in the series, but I can't put my finger on the exact quote.