Hypothesis
Alpha entrainment by tACS, reported with an anterior/posterior two-electrode montage, would be robust enough to survive scaling up to a multi-electrode montage and changes in the participants' background task, raising subsequent alpha power.
What failed
A multi-electrode analogue of the earlier anterior/posterior stimulation, given while background tasks varied, did not reproduce the reported alpha entrainment: subsequent alpha power was not modulated.
Broken assumption
tACS alpha after-effects are robust to task context and to moving from two-electrode to multi-electrode montages.
Why
The abstract gives no mechanism; the authors say more work is needed to define when tACS alpha entrainment applies.
From the published paper: Fekete et al. Frontiers in Neuroscience, 2018. ↗CC BY