In certain configurations you can do a pin mod in conjunction with a volt mod to get higher speeds. Whether it is stable...
For instance, if you have a Celeron 2GHz or Pentium 4 2.0A in a Dell that officially supports 533MHz FSB, then you can break a pin off and the system will detect it as a 2.66GHz chip. A Celeron 800 will become a Celeron 1066 and a P3 550 will become a P3 733 on socket 370 boards that support 133MHz FSB.
Of course, YMMV. Some BIOSes get stupid if the CPU isn't in their microcode meaning a P3 550@733 may be fine because a P3 733 exists and a P4 2.0A@2.66 may be fine because the 2.66 exists, but the Celeron 1066 and 2.66 (Northwood, not Prescott) didn't exist thus may not work if the BIOS doesn't like it. Also, if the CPU just can't run stable at that overclocked speed then you're SOL.