Most concurrent data structures do not guarantee consistent traversal if the data structure is modified during traversal. This is, in fact, the case with most mutable collections, too. Concurrent tries are special in the sense that they allow you to modify the trie being traversed itself. The modifications are only visible in the subsequent traversal. This holds both for sequential concurrent tries and their parallel counterparts. The only difference between the two is that the former traverses its elements sequentially, whereas the latter does so in parallel.
This is a companion discussion topic for the original entry at http://docs.scala-lang.org//overviews/parallel-collections/ctries.html