This is with Processing 1.0.1 on Windows XP.

Repro:

1. Load Processing with a new sketch.
2. Add some code that defines a class:

void setup()
{
size(400,400);
}

void draw()
{
background(0);
stroke(255);
Foo f = new Foo(10);
point(f.getX(), f.getX());
}

class Foo
{
int x;
Foo(int X) { x = X; }
int getX() { return x; }
}

3. Run it to verify that it works.
4. Save As, and enter then name "Foo".
5. Exit Processing.
6. Navigate to My Documents\Processing\Foo, and double-click Foo.pde
7. Click the Run button.

Result:

"The constructor Foo(int) is undefined" shows up in the PDE's output bar above the
console area.

I can see why, in hindsight, this would happen. But it would be nice if the error
message were clearer ("Error: class Foo has same name as applet" or whatever), or if
the PDE issued a warning in the Save As process if you happen to choose that name.

*** Bug 1294 has been marked as a duplicate of this bug. ***