Caffeine is well known as a stimulant that has benefits for athletic performance (although the response and ergogenic potential is individual).
A typical dosage for performance is between 3-6mg/kg of body mass.
However, cups of coffee can have a significant difference in caffeine content.
Homebrews: 80-120mg of caffeine
Speciality brews: May have upwards of 500mg
A bigger issue for athletes targeting specific doses is that caffeine blends of speciality cups from your well-known coffee chains can vary daily.
So that caffiene hit wanted before training may have huge differences in each cup.
The study above investigated the caffeine content of the same brew, from the same shop for size days in a row.
The caffeine content of each blend ranged between 259-564mg for the same drink on different days.
For a 70kg athlete - 564mg would be much higher than the top end of recommended ranges.
Further Advanced Reading: