It wasn't until the 60's the error coins became popular. Proof of this is the fact that the majority of older error coins are well worn.
There have been a few publications about error coins. One by Alan Herbert and one by Frank G. Spadone both on USA error coins.
Every edition of Charlton Coin Catalogue of Canadian Coins usually also has a few listing of some varieties of errors caused by die variations in the rear of the books.
I got into selling and collecting error coins only because of their rarity.To me to collect a coin lets say a 1921 Canadian silver 5 cent coin (mainly because of its rarity). Now if you saw two for sale, one with an error and one without. Which would be the rarest?.....right the error one! My point made I hope!
Now lets say you had a collection of Canadian 1 cent coins and I had a collection of the same coins in the same condition but all error coins. Who's set would be rarer? The rarer an item the more value it demands,right?
In my opinion the off struck coins are the rarest, being followed by oddly mint damaged coins. The ones that are easiest to find are the die cracks,double dates,doubling of the figures, doubling of the printed letters. Not that doesn't make them any less rare.They just seem to be more of them.I think the error larger errors are usually picked up at the mints by the employees and culled out.Where the smaller errors are harder to see,therefore they are more apt to be missed.
To find an error coins you don't always need a large quantity of coins.Some of the nicest error coins come from plain old pocket change. I have found more error coins out of pocket change than I ever found in mint rolls.But one thing you do need is a loupe. A small one eye magnifying glass like the jewelers use. Preferably X15 but a X10 does a good job as well. I use a X10 eye measuring gauge as its more open with glass 1/2 the way up and you don't need a lot of light to see through it.Also it has the measuring gauge on it incase you need to measure the diameter etc. of a coin.
One would be surprised at the actual value of their pocket change if they checked every coin before parting with it.
Just as one example: I found a newer penny one day in my pocket with a groove cut in it, all the way around, and right beside the denticles/rim. I listed it on Ebay and thanks to another Ebayer who told me how this had come about,I got approx. $30us for it. This error is cased at the mint by the blade the machine uses to trim off the excess at the ends of the rolls.And it only occurs to very few coins as the blade is only changed every few thousand rolls or so and only occurs at certain mints.
UPDATE : Sept.8,2007
Just for fun,check all your nickles with Queen Elizabeth's second or mid life portrait on them..check for bugs on beavers back and tail,die crack through the last 9 in 1999,bump or bug on Queen's nose right between her eyes,check for extra metal under her pendent on all the P coins,check for doubling of the Queen's front and back,extra metal anywhere on the coin(especially letters and numbers),die cracks by the beaver's tail,missing denticles,planchet flaws,etc.
Please keep checking this guide as I am always updating my guides as I learn new things.I just hope this has helped some regarding error coins.
Try the following link for more information on error coins try this site: coinscan.com/err/dcl.html
Another great book that list a lot of errors od canada coins is Charest book of Canadian Coins..which is avauilable on the web..just do a search...charest.qc.ca/book
UPDATE: May 30,2008
Canadian 1c cent coin errors
1965-79 when looking, look for the following commonest errors:
1) doubling of the Queen's forehead & nose
2) doubling of the Queen's back
3) doubling of date or parts of date
4) rotated dies(when reverse and obverse don't line up)
Please note the difference detween a DIE CRACK and PLANCHET CRACK .
Die cracks end up as raised lines on coins where planchet flaws are actual grooves in the surface on coins.
I have noted a lot of amateur slouths listing planchet flaws as die cracks....beware.
Interesting find 1990 Canadian 5c - HORIZONTAL LINES UNDER CENTER OF BEAVER INSTEAD OF FUR...may become a variety???