Normally, hitting Shift d erases everything from the cursor to the end of the line, but sometimes it just deletes one character backwards.
This has happened more than once, but a recent example is that I was experimenting with the
c operator, because it can be pretty nifty in terms of editing text and automatically going into insert mode.
Something that I pressed in the process triggered a change in behavior with the Shift d command, and it would be interesting to know what was going on.
I'd give more information if I could.