When you kill Bowser at the wrong time.
From http://tasvideos.org/forum/viewtopic.php?t=9844 (not my work)
"If anyone's interested in the technical details, Bowser's y velocity is at $d4. On a small hop, this gets set to 240 (1 px/frame upward velocity). When Bowser is killed, it gets set to 204 (3.3px/frame upward). When the Quick Drop is performed correctly, Bowser's y velocity gets set to 204 when he's killed, but continues his pattern for 1 frame longer than he should - so his y velocity gets overwritten with the much slower 240 speed.
What happens when Bowser is killed just before a big jump? Well...it's interesting. "
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