Around 5,200 utility patents issued on November 6, 2012. I was surprised to find that at least 76 of those had been abandoned at some point during prosecution and later revived and then patent. Although only about 1.5% of the patents, the number is still surprising. Of course, revival has become somewhat regularlized these days and is done with little consequence beyond a substantial fee ($1,890 plus any late fees) and loss of some PTA. In fact, the USPTO has created an automated computer system to grant petitions to revive so long as they are submitted and signed (electronically) by a registered patent practitioner. Further, the current thinking is that a patent cannot be later challenged for improper revival because that issue does not serve as a condition for patentability that qualifies as a defense under 35 U.S.C. § 282.
Even though my sample here is quite small, it still included many patent-industry leaders such as Fujitsu, GlaxoSmithKline, Texas Instruments, Carl Zeiss, Wi-LAN, Philips, IBM, Johnson Control, University of Texas, Illinois Tool Works, Intel, 3M, Seagate, University of Maryland, EMC, Sharp, LG, Kodak, Microsoft, FEI, etc. Each of these entities now has at least one new patent that was accidentally abandoned during the application processing and then later revived.
From my analysis of these cases, I do not perceive any attempts to game the system. Rather, the abandonments seem to be truly unintentional misses by offices and attorneys managing hundreds of cases. Luckily for the parties involved, the revival process is almost as easy as the abandonment.