تَأَخَّرَتِ الْقَافِلَةُ، وَلَكِنَّهَا لَمْ تَضِعْ.
The caravan was late, but it was not lost.
The second clause does not retract the first. It blocks an inference listeners could otherwise make without adequate grounds. The usefulness of corrective contrast depends on a plausible mistaken expectation; two different reports alone do not warrant lakinna.
When two affirmative propositions are simply related, wāw may instead be appropriate. The exact relation is determined by preceding discourse, not by particles in isolation.