I believe that the two pieces of textual evidence from The Metamorphosis that best illustrate the theme that life is unpredictable are:
1) "One morning, when Gregor Samsa woke from troubled dreams, he found himself transformed in his bed into a horrible vermin." [here, Gregor wakes up from his sleep to see himself turned into a bug, a very unpredictable thing]
3) "He really ought to have expected things to have changed, but still, still, was that really his father?" [the change his father went through is also an example of how life is unpredictable]