In my experience, condition rarity key dates are a largely unreported segment of coin collecting. A condition rarity is a coin that is otherwise common, but is extremely tough to find in certain conditions. This type of coin is generally rare in uncirculated condition, but readily available in circulated grades. A well known example is the 1944 red cent. It can be quite easily acquired in all grades, but any example with full red color is extremely difficult to acquire, and will fetch significant prices. Many condition rarity coins do not directly correspond between the listed prices and the market prices, and are not as obvious to the average collector. What follows is a list of condition rarities I'm familiar with (admittedly not a comprehensive list). If you know of any others, please send them my way and I will add them to the list (and attribute them to you). An important note is that some of the rarities listed here may cease to be rarities in a few years. The 1989 5 cent in MS-65 is a rarity now, but if a few more examples come to market, prices should be expected to drop. I'm not saying that will happen, but it's always a possibility -- moreso with modern coins than classics.
A few interesting ones.
1970 5 cents
1970 10 cents
1991 25 cents
These are interesting because they are the opposite of a condition rarity. They are almost always found in Mint-state condition, but are next to impossible to discover in lower circulated grades. These coins were recognized for their low mintages in the year of issue and were extensively hoarded. Very few ever spent any significant time in circulation. Worn examples of these coins may be an interesting conversation starter, but they do not command a premium price; it doesn't take much effort to take a MS example and bring it down a few grades.
True condition rarities
Newfoundland red small cents. Low grades can be picked up for a buck or two, but you hardly see MS-64 or higher examples on eBay.
1953 SF 1 cent, MS-65 full red.
1983 1 cent in MS-65.
1990 cent in MS-65 red or better. It was a lower mintage year, but with a mintage high enough that people didn't bother to hoard, a common element in these condition rarity coins.
MS-65 10 cent, 25 cent and $1 coins from 1968-present. I am painting with a broad brush here, but there is a clear division in Canadian coin collecting. Generally speaking, Canadian coins can be divided into the silver era from 1858-1967, and the modern era. Many collections stop at 1967, and the nickel examples from later years are often tossed into junk piles or spent. Shortage of collectors saving them, and the tendency for Nickel coins to pick up bagmarks easily, makes the nickel series very tough to collect in the highest grades.
1959-61 Silver dollars in MS-64. These were largely considered common in comparison to the earlier silver dollars, and were given the bulk bucket treatment by dealers for many decades. Finding one without a baggy surface is a challenge.
1965 Type 2 and Type 4 in MS-64. The Type 5 gets all the hype, but it's probably easier to find in MS-64 condition than these two examples. Again they are another "common" coin that has been tossed aside for years leaving very few survivors in nice condition.
Many collectors will claim that these coins are very common, and that it hasn't been worthwhile for people with hoards of these coins to have them certified. I have trouble believing this statement given the prices many of these coins achieve on eBay when they appear. While some are undoubtedly still out there, I believe strongly that many of these coins will hold up as genuine rarities.
Thanks again for taking the time to read my guide. If you've enjoyed it, be sure to read my other guides, and click "Yes" below to let me know you've found it helpful. Don't hesitate to forward any examples you know of, and I'll add them to the list. These coins can be both challenges, as well as bargains, best of luck and happy hunting.