I still find it difficult to criticise PC/JS for the Whitehurst trade, the guy has all the physical tools, it's hard to get a good reading on a guy without having him in the system first though.
And I never felt we overpaid either. QBs are never cheap, and we gave up a 2011 third round pick for a trade in 2010 - a pick that ended up being 89th because we made the playoffs. Oh, and although we traded seconds, we picked up Golden Tate with that 2nd in the end.
Jury is still out on the Harvin trade, but you could also say that about Bruce Irvin and Christine Michael too. I have faith that all 3 will work out for the best in the end.
I would say rather than "biggest mistake" is that I still don't have great faith in the FO's ability to draft WRs, Tate is the only success story and he took two years to finally get there. Durham and Harper never worked out. Additionally we have traded for/picked up high priced WRs Rice and Harvin, and one proved to be injury prone, the other has only played 3 games for us so far. Fortunately we have mitigated that somewhat with an ability to pick up undrafted receivers like Baldwin, Kearse and Lockette, so it's not the biggest issue in the world.