Debian stable.
Build from 2019-01-28 (cad1016c20b5)

A second keyed frame, when removed, does not fully clear the keyframe.

More detailed:
Keying a field (VSE -> new Color track -> Opacity field) to 1, then in a later frame keying that one to 0, then removing with Alt-I (over the field), will brown the field and remove the keyframe from the graph, but the values still change when scrubbing (and they all appear brown, between the original 1.0 keyframe and the removed keyframe).

A couple Notes:

1. I-Key keys the field and makes it yellowish-green. Alt-I causes the field to change brown (which seems to be the color for manually-changed but non-keyed values. ie. it should revert when scrubbing. However, all frame positions between a keyed frame and this deleted-keyframe show as brown. Neat.
2. Saving and reloading with a fresh blender instance clears this. The fields all appear gray again and their values don't change.

Steps to reproduce (because, if at first you don't succeed, try over and over to make sure you can't succeed):

1. New blend.
2. Change to VSE and add a Color track.
3. Go somewhere midway through track and hit I over the opacity field (to key it to 1.0).
4. Go later and I-key the field to 0.0.
5. Alt-I to clear the field makes it brown but scrubbing shows the values changing, as if it were still keyed.

Might be related to:
https://developer.blender.org/T61148