MVC3 Wrong form submits

Question

From every page of a website I'm making, you're able to sign in. On pages with other forms, I get the following error after submitting: "Child actions are not allowed to perform redirect actions." All forms worked fine until I added the Sign In. In addition, the Sign In works correctly when you use it. The error only appears if I submit a different form. I've set a breakpoint and I've watched what happens when I hit the other submit. For some reason, the SignIn Post ActionResult is trying to run. Any help would be appreciated.

Error:

Child actions are not allowed to perform redirect actions. Line 67: @{Html.RenderAction("SignIn", "Account");}

Sign In View

@using (Html.BeginForm("SignIn", "Account", FormMethod.Post))
{
    <input class="signInSubmit" type="submit" name="submitButton" value="" />
}

Another Form in View

@using (Html.BeginForm("Confirm", "Cart", null, FormMethod.Post, new { @id = "productDetailsForm" }))
{
    <input class="addToCart" type="submit" name="submit" value="" />
}

Sign In Controller

// GET: /Account/SignIn
public ActionResult SignIn()
{
    return PartialView();
}

// POST: /Account/SignIn
[HttpPost]
public ActionResult SignIn(Customer customer)
{
    try
    {
        //Stuff is here
        return RedirectToAction("Index", "Home");
    }
    catch
    {
        return RedirectToAction("Registration");
    }
}

Other form Controller

// GET: /Cart/
public ActionResult Index()
{
  CartViewModel cart = getCart();
  return View(cart);
}

//POST: Cart/Confirm
[HttpPost]
public ActionResult Confirm(int productID, bool certs, int quantity)
{
  CartItemViewModel viewModel = new CartItemViewModel
  {
    Item = productRep.GetProductByID(productID),
    Certs = certs,
    Quantity = quantity
  };
  return View(viewModel);
}

HTML Source Code

<form action="/Account/SignIn" method="post">
    <input class="signInSubmit" type="submit" value="" />
</form>
<form action="/Cart/Confirm" id="productDetailsForm" method="post">
    <input class="addToCart" type="submit" value="" />
</form>

Answer 1

My solution

I renamed the POST ActionResult to anything other than SignIn. Does anyone understand why this works?

AccountController

// POST: /Account/Signed
[HttpPost]
public ActionResult Signed(Customer customer)
{
    try
    {
        //Stuff is here
        return RedirectToAction("Index", "Home");
    }
    catch
    {
        return RedirectToAction("Registration");
    }
}