When a coin is born, if the initial difficulty to get a block is too low, you can get a ton of coins (blocks) really fast if you get in early. The coin is instantly mined.
Similarly, when a coin is mined to almost completion in a very short period of time some people consider it's been instamined as well.
FTC and QRK are two big examples in each category.
lol ... 6 months of mining for QRK is hardly an instamine, try Blackcoin with just 6 days of mining ...