As a reminder, a unilateral contract is where an offeree accepts through performance.  A common example that professors like to use is: A says to B, “If you walk across the Brooklyn Bridge, I’ll pay you $100.”  To get paid, B must cross the bridge.  Promising to cross the bridge is not enough.

Our traditional rule is that A can revoke this offer until B completes performance.  So if B starts walking across the bridge, A can say, “I take back my offer!”  Now B does not get paid even if she crosses the bridge because A revoked the offer.

The modern rule is different – – unilateral contracts cannot be revoked once performance begins. That is, if B starts performing, A cannot revoke the offer.  In the above example, if B is crossing the bridge,  A cannot revoke the offer.

Below is an older video which discusses terminating offers. Around the 3:50 mark the video  discusses revoking unilateral contracts.