Anything can/might happen
Definition 1 of 1
Pro Tip 1/3Expect Uncertainty
This phrase implies that the future is uncertain and outcomes cannot be predicted. 
He's taking a huge risk with his business, anything can happen.
Pro Tip 2/3Embrace Possibilities
Encourages an open mindset for all possible outcomes, especially in changing situations. 
She's going into the audition without expectations because anything can happen.
Pro Tip 3/3Informal Tone
It's a casual phrase more likely to be used in everyday speech rather than in formal writing. 
At a start-up launch, anything might happen, so adapt quickly.