The rule that a block may not contain more than 20,000 signature operations. Satoshi was concerned that if this rule change was publicly known, someone would try to make some really expensive blocks before the new rule was scheduled to take effect. However, nowadays a rule change such as this would be very controversial, and this particular rule might cause problems in the future because the limit is quite low.
Interesting, I didn't know that, thanks theymos
Is nice to know we have all the bases covered, because everything unlimited in this world gets abused with enough time.