Prompted by some curiosity online, I checked my own recent purchases of the Lego spiral staircases, I decided to check and see if indeed I was sold defective pieces, since I wasn't aware while assembling their respective sets that there was any defect.

Upon closer inspection, there was.

On the left in the picture below, is the set of "normal" stairs, at their 'widest'. And on the right is a set of "defective" stairs, similarly at their 'widest'. Notice how there is a slight gap in the defective set, versus the normal set which has a significant overlap.


The overlap in a normal set of stairs is about 3mm, whereas the gap in the defective set is between 1mm and 2mm.

Looking at the underside of the pieces, we can see the difference between the defective and normal pieces (defective has now switched sides and is on the left, with the normal stair on the right):

The tops of the stairs, on the other hand, appear identical:

And of course the reason it matters is the orientation of the 3 notches underneath. The middle notch is always apparently centered, however the side notches are just off a little, assumedly to ensure a small overlap.

For the purposes of record:
The stairs I purchased were from set 4706 Forbidden Corridor and 4709 Hogwart's Castle I purchased both in late 2001, and only 4709 contained the defective stairs. Each came with 16 stair pieces, and of the 16 in 4709, 6 were defective, the other 10 were normal.

There are also two numbers printed on the bottom of each piece. One seems to be a part number (consistant on all stair pieces), while the other number differs. Within 4709, all matching numbers were consistantly defective or not. IE the three steps with a 6-02 were all defective, and the four labelled 5-02 were normal. However, there were some with matching numbers (between sets) which were both normal and defective:

Set	Part Number (Quantity)
4706	1-01 (1)	2-01 (1)			5-01 (2)		7-01 (1)
		2-02 (2)			5-02 (3)	6-02 (1)		8-02 (5)
4709	1-02 (5)	2-02 (2)	3-02 (1)		5-02 (4)	6-02 (3)		8-02 (1)
4706 (Data from Pawel)	1-01 (2)	2-01 (1)	3-01 (1)		5-01 (1)	6-01 (3)	7-01 (1)
	1-02 (2)	2-02 (1)	3-02 (2)	4-02 (1)				8-02 (1)
4706 (Data from Adrian)	1-01 (1)					6-01 (1)		8-01 (1)
	1-02 (2)	2-02 (2)	3-02 (2)	4-02 (1)	5-02 (3)	6-02 (1)		8-02 (2)
Totals
4706 (3)	1-01 (4)	2-01 (2)	3-01 (1)	4-01 (0)	5-01 (3)	6-01 (4)	7-01 (2)	8-01 (1)
	1-02 (4)	2-02 (5)	3-02 (4)	4-02 (2)	5-02 (6)	6-02 (2)	7-02 (0)	8-02 (8)
4709 (1)	1-02 (5)	2-02 (2)	3-02 (1)	4-02 (0)	5-02 (4)	6-02 (3)	7-02 (0)	8-02 (1)
Key:	Normal				Defective