During the fight between Minato Namikaze and Tobi (in episode 248 The Fourth Hokage's Death Match of the Naruto: Shippuden anime, or chapter 502 and 503 of the manga), Minato escaped from Tobi's Kamui, despite having been caught in the technique unexpectedly.
3 minutes and a few seconds later into the episode, the two ran towards each other. At an extremely close distance, Minato pulled off a Rasengan. Considering the bright aura and the sound it makes, I'm pretty sure Tobi noticed it.
Tobi let the kunai that Minato had thrown pass through him. So instead of using Kamui to let Minato phase through him, why did Tobi keep going? Minato had already escaped from Kamui once, and that was when he was caught unexpectedly.