Chazal teach us that three things arrive unexpectedly – Mashiach, a metziah (a find), and a scorpion. 

What is the connection between these three?  If one is zoche (merits), the arrival of Mashiach will be like a pleasant but unexpected appearance of a metziah – he will rejoice and it will be good for him.  But if he doesn’t merit, Mashiach’s coming will be sudden and harsh, like the unexpected pain from the bite of a scorpion! ~ The Maharsha