In the context of the ATF "Fast & Furious" investigation, some of the Republicans are floating the notion that there has been improper preparation of percipient witnesses. Specifically, it's allged that the DOJ has built an internal database of documents and witness statements that all their witnesses could access and that many did.
I'm unaware of any legal authorities holding that that's per se improper witness preparation, but depending on the intent and the actual behavior it theoretically could be an improper aligning of the testimony in an extreme case. Certainly, in the normal business case for a private practice lawyer's client, a lawyer would normally have a party-witness read the relevant documents and emails -- including emails the witness didn't write -- before testifying at deposition. Should the result be any different for government lawyers working with government witnesses who will be appearing before a governmental oversight? (As will be obvious from my comments above, I find the conclusions in this article to be overblown and overstated, but it does indicate that the issue won't be going away soon._