The SCUD is little more than an updated V-2. The warhead stays attached to the rocket body for the entire flight. Like the V-2, SCUDS sometimes break up as they approach the target. The Patriot PAC II system used in the Gulf War couldn't discriminate the warhead from the debris. It seems there was also a fusing timing error of a few microseconds that caused the Patriot to explode a little late. It blew up the empty rocket bodies but left the warhead intact.

I read some classified reports on the barracks incident. Since I don't know if they've been declassified or not, I won't mention anything specific. That was a SCUD warhead that killed the people in the barracks. I do know this part is unclassified: The Patriot missed because its computer hadn't been rebooted for a long time and timing errors had crept into the calculations. The Patriot missile specifications called for the system computer to be rebooted at least every 14 hours. The Patriot battery that missed the SCUD had been running continuously for about 100 hours. Here's an excerpt from the book "Computer Related Risks" by Peter G. Neumann:

"Patriot clock drift. During the Persian Gulf war, the Patriot system was initially touted as highly successful. In subsequent analysis, the estimates were severely downgraded, from about 95 percent to about 13 percent (or possibly less, according to MIT's Ted Postol; see SEN 17, 2). The system had been designed to work under a much less stringent environment than that in which it was actually used in the war. The clock drift over a 100-hour period (which resulted in a tracking error of 678 meters) was blamed for the Patriot missile missing the SCUD missile that hit an American military barracks in Dhahran, killing 29 and injuring 97. However, the blame can be distributed - for example, among the original requirements (14-hour missions), clock precision, lack of system adaptability, extenunating operational conditions, and inadequate risks analysis (SEN, 16, 3; 16, 4). Other reports suggest that, even over the 14-hour duty cycle, the results were inadequate. A later report stated that the software used two different and unequal versions of the number 0.1 - in 24-bit and 48-bit representations (SEN 18, 1, 25). (To illustrate the discrepancy, the decimal number 0.1 has an endlessly repeating binary representation 0.0001100110011... Thus, two different representations truncated at different lengths are not identical - even in their floating-point representations.)"

page 34
"Computer Related Risks"
Peter G. Neumann
ACM Press