Windows example:

test1.d:
module test;

import std.stdio;
import std.process;

void main()
{
    system("echo > test.exe");
    auto file = File("test.exe", "r");
    system("dmd test2.d -oftest.exe");
}

test2.d:
module test2;

import std.string;
string mixMe()
{
    string res;
    foreach (i; 0 .. 3_000)
        res ~= xformat("int i_%s;", i);
    return res;
}

mixin(mixMe());

void main()
{
}

$ rdmd test1.d
OPTLINK (R) for Win32  Release 8.00.12
Copyright (C) Digital Mars 1989-2010  All rights reserved.
http://www.digitalmars.com/ctg/optlink.html
OPTLINK : Error 3: Cannot Create File test.exe
--- errorlevel 1

test2.d demonstrates a module that takes a longer while to compile. test1.d creates a phony test.exe, then opens it in read-mode to lock it. Then it attempts to compile test2.d and write over test.exe.

DMD will first compile test2.d and only then attempt to write to test.exe and fail. This can be a considerate waste of time, the compiler should check if the output location is writable *before* attempting to compile.