Trisha Gao's Library tagged anova →  View Popular

But note that in this scenario there would be
no main effects for either rows or columns
(M_row1=M_row2=7.5 and
M_col1=M_col2=7.5),
notwithstanding that drugs A and B are obviously having an effect when
presented separately. The moral of this story is that the interpretation of the
presence or absence of main effects in a two-way ANOVA is not always simple and
straightforward. More of this later.