there's many a slip 'twixt cup and lip

Meaning

1. nothing is completely certain until it really happens because things can easily go wrong

From the same entry

• let slip
• let slip
• slip your mind
• slip one over on
• slip through the net
• out of gear
• roll off the tongue
• slip away
• slip out
• slip up
• give the slip
• a slip of a boy, girl