Yeah. Overvolting can screw with the transistors (which isn't something you can see just looking at the CPU), the core can be squashed, the CPU could be manufactured outside normal tolerances and die early, and so on.
If you didn't see any burn marks, they're probably lying. Yell at them.
DEC Pentium X2 5200+ w/ HyperCache (Ezra core)
Asus M7NCD-MAX3 (OPTi Vendetta 82C760)
6x Generic 32MB PC2700 RDRAM (50ns SIMMs)
2x nVidia Millennium X1800 Duo (SLI mode)
12x (daisy-chained) Quantum Medalist 180GXP (w/ separate SCSI-1 adapter)
"Oh, you started a Rube Goldberg machine. A Rube Goldberg machine... called JUSTICE." <a href='http://www.boomspeed.com/old_camper/amdforumsircchat.ht