Steps to reproduce:
1. Open attached ‘BasicPlayableBehaviour.zip' Unity project
2. Open scene ‘Test'
3. Enter Play mode

Expected results:
Every BasicPlayableBehaviour clip OnBehaviourPlay in the Timeline is fired
Actual results:
One or more BasicPlayableBehaviour clips are skipped

Reproduced in: 2017.3.0a7, 2017.2.0b11, 2017.1.1p1, 2017.1.0b3

Note: Entering the Play mode while paused (or clicking Step through the duration of the timeline) seems to produce different results, sometimes a different clip is skipped or none are skipped at all.

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By design:
Timeline evaluates at discrete points based on the frame rate. It is possible timeline may not evaluate at a point where the time overlaps a clip.