I think that one of the best examples for this not so rare occurence exist today in this EURUSD chart.
The most big picture shows what seems to be an which should lead to a down side movement at the time when the neck line will be broken (if gets there)
But a more carefull look shows a different picture.
1st of all, the main problem for the head and shoulder is that this should be a reverse signal, and for a reverse signal should be a strong trend to reverse, in order to become meaningfull. Here I can't see this. This just could reverse it self (at most).
Then if we keep looking we can find that the uptrend has not failure, constantly find higher lows, and highs, but the last one.
Even so, in this last time, the lack of higher high do not mean that won't be done in the future, because we can still observe an upside trend with a fresh new high inside it.
Of course that there is still chance for a pullback to retrace the las upside movement, but we still won't see conclusive evidence that the uptrend will be finished until the last low will be broken. Even more, before that, at 1.3109-1.2997 levels the price shows chances for an upside reversal, of course, the higher the better.
Thats why i think the weight we give the evidence, makes the difference. To me, there still are chances to longs and not shorts under the actual evidence.