Stats - type I error

A type I error is rejecting the null hypothesis when it is true. A two-sided, often known as 'two-tailed' hypothesis test is a test that looks for a change in the population parameter, not just an increase or decrease (such tests are called 'one-tailed' tests). From this, a "two-sided type I error" is the event of rejecting the null hypothesis when it is true in a two-tailed hypothesis test.

Hope that helps.