According to the Federal Reserve:

Known counterfeit notes in denominations of $50 and $100 have increased 60%.
In 1990 alone, over 150 counterfeit operations were shut down.

Counterfeit travelers checks, credit cards, government checks and ID cards are also on the rise.

The commonly used "counterfeit detection pens", which perform a simple chemical test, are easily fooled with products found in many households.

Looking for security features with the naked eye or with a magnifying glass is time-consuming, prone to error and obviously dependent on lighting conditions.