The following unit test fails (i == 2):

unittest
{
    import std.array;
    import std.random;
    import std.range;

    int someFun()
    {
        return uniform(0, 2000);
    }

    int i = 0;

    const arr = generate!(() {
        ++i;
        return someFun();
    }).take(1).array;

    assert(arr.length == 1);
    assert(i == 1, "generate called fun more than once");
}