Skip-number: Card sets appeal, above all other factors, to the collector instinct that is very strong in some people. To capitalize on this, it is thought that some sets have holes in the consecutive numbering intentionally punched in them. 1933 Goudey is one example, the idea being that an unsuspecting kid would forever buy pack after pack to find the one card he was missing for his set.
Sometimes there is just a mistake. 1995 Select Certified baseball is missing card #18. It was pulled from the set to make way for a Cal Ripken, Jr. card numbered #2131. Other cases involve disputes over rights; the Topps 1953 baseball set is missing six cards because Topps could not get the players signatures on contracts to reproduce their likenesses. Interestingly, Topps had their portraits produced in anticipation of that set, and those pictures sat unused for decades before being auctioned off in the mid-'80s. Another such example is the 1999 Upper Deck Century Legends set, where they could not arrive at agreements with Pete Rose, Sandy Koufax, or the estate of Joe DiMaggio. Topps "retired" card #7 in their base set in honor of Mickey Mantle starting with the 1996 set, so Topps sets will forever after be skip-numbered. However, Topps brought back Mantle cards, and then re-retired them again.