Before revision:  When regressors containing lags and leads of treatment are added, the OLS coefficient corresponding to a given lead or lag picks up spurious terms consisting of treatment effects from other periods . Therefore, estimates from these commonly used models are not causally interpretable .
After revision:  When regressors containing lags and leads of treatment are added, the OLS coefficient corresponding to a given lead or lag picks up spurious terms consisting of treatment effects from other periods , and apparent pretrends can arise solely from treatment effects heterogeneity .