Depends which efficient market hypothesis (EMH) you believe.
I believe in semi-strong EMH.
The market isn't all knowing and you can beat it, by trying to predict the next announcement.
I learnt from my last experience with GMC, that even with an executed and notarised agreement (with Pak Marthen), it is risky predicting a positive announcement with GMC despite all the imminent updates that appear to support it.
Upon publication that Pak Marthen was no longer GMC dropped 85% thereby supporting semi-strong EMH.
Similarly, on publication of any bad news going forward relative to expectations, semi-strong EMH would indicate a drop in share price.
The key here is relative to expectations. You just need to look at what the popular sentiment is on here (by reading the posts with the most likes) to gauge what expectations shareholders are pricing in.