Okay, so I've been working through the various pricing and discounting schemes that Zen Cart supports, and I think I mostly understand how it works by now. But there's one case in the collection of demo products that comes with a fresh Zen Cart 1.3.8a install that has me seriously scratching my head:

ID# 78 - TEST 25% special 10% Sale Attribute Priced

The first two of these head-scratchers I was able to figure out myself.

On the freshly-installed system, this product was shown as follows:
--------------------------------------------------------------------------
TEST 25% special 10% Sale Attribute Priced
Starting at: $150.00(struck thru) $112.50
Save: 25% off

Please Choose:

Color [radio buttons]:
Orange ( +$37.50 )
Yellow ( +$56.25 )
Red ( +$75.00 )

Size [pulldown]:
Small ( +$37.50 )
Medium ( +$56.25 )
Large ( +$75.00 )

Gift Options [checkboxes]:
Collector's Tin ( +$37.50 )
Autographed Memorabilia Card ( +$56.25 )
Wrapping ( +$75.00 )
--------------------------------------------------------------------------

The two obvious questions about this initial configuration were:

1. Where's my 10% sale discount?
2. Why is the base price $150 (undiscounted) or $112.50 (discounted)? A small, orange thingie without any gift options should be $100 (undiscounted) or $75 (discounted).

After a little digging, I discovered that the "10% Off Sale" was initially created with an end date of 2/21/2008, and that the Gift Options weren't actually configured as "optional," i.e., they were marked as contributing to the product's base price.

After I re-enabled the "10% Off Sale," the product pricing showed up as:
--------------------------------------------------------------------------
Starting at: $150.00(struck thru) $112.50(struck thru)
Sale: $101.25
Save: 33% off
--------------------------------------------------------------------------

So far so good. But...

When I made the Gift Options truly optional (by X-ing out the violet-colored "Base Price" boxes in the Attribute Controller for that attribute), the undiscounted base price changed to $100 as expected, but the special and sale prices remained unchanged. And worse, the sale price is now higher than the base price, and the savings percentage is now negative!
--------------------------------------------------------------------------
Starting at: $100.00(struck thru) $112.50(struck thru)
Sale: $101.25
Save: -1% off
--------------------------------------------------------------------------

Huh? It sure seems like the special price should now be $75, since the special price is defined as 25% off the product price.

Although this feels like a bug, I can't imagine that I'm the first to find it. And I've looked through the list of known bugs and haven't found anything that applies. So I'm probably doing something wrong, but I can't see it.

What am I missing?

Thanks,

Michael