Scenario,: this code does not work without the dummy line:

@using umbraco.MacroEngines
@{
    var previous = Model.Previous();
    var nextProduct = Model.Next();
}
@if (previous != null || nextProduct != null)
{
    <div id="prev-next">
        @if (previous != null)
        {
            <a href="@previous.NiceUrl">
                ..
            </a>
        }
        @if (nextProduct != null)
        {
            var dummy = nextProduct.Id; // this line fixes the bug
            <a href="@nextProduct.NiceUrl">
        ..
            </a>
        }
    </div>
}

Without the var dummy=... line the @nextProduct.NiceUrl returns #