debug statements are allowed to violate @safe, pure, @nogc and nothrow (allthough the latter is not implemented right now). But they influence the attribute interference:

----------------------------------------------

void main() pure /* nothrow */ @safe @nogc
{
    debug foo();
    bar!()();
}

void foo() @system
{
    // Just to be sure its neither @nogc, pure or nothrow
    __gshared int counter = 0;

    if (counter++)
        throw new Exception(new immutable(char)[counter]);
}

void bar()()
{
    debug foo();
}

----------------------------------------------

..\debugging.d(4): Error: @safe function D main cannot call @system function debugging.bar!().bar
..\debugging.d(16):        debugging.bar!().bar is declared here
..\debugging.d(4): Error: @nogc function D main cannot call non-@nogc function debugging.bar!().bar

----------------------------------------------

The debug statement caused bar to become @system, non-@nogc and non-nothrow.