  In light of recent work of the third author, we revisit a classic example
given by Fritz John of a semi-linear wave equation which exhibits finite in
time blow up for all compactly supported data. We present the construction of
future global solutions from asymptotic data given in arXiv:2204.12870(2022)
for this specific example, and clarify the relation of this result of Yu to
John's theorem. Furthermore we present a novel blow up result for finite energy
solutions satisfying a sign condition due to the first author, and invoke this
result to show that the constructed backwards in time solutions blow up in the
past.
