The condom boxes say there's a 2% failure rate. That's 2% per year. Considering that most people are sexually active for far more than one year, that 2% rate is completely meaningless.
Do people actually interpret that 2% to mean 2% per lifetime? If I didn't know what it meant, I'd interpret it as 2% per intercourse.
Yes, unfortunately, a large number of the people I've talked to do interpret it as 2% per lifetime.
P/S/A: There are single sentences which can create life-changing amounts of difference.