From a fan of the Brief editor.
A few people in the early days of the Internet Explorer group used the Brief editor, which uses Ctrl+Backspace as the shortcut key to delete the previous word, and they liked it so much that one of them added it to the autocomplete handler. Therefore, any edit control that uses
SHAutoComplete will gain this secret Ctrl+Backspace hotkey.
This is one of those scary “rogue features”. There’s no spec, there’s no testing, it’s just a developer who spent some time whacking a feature into the product because he liked it. Sure, rogue features can often become useful, but they also create a lot of liability. Imagine if a rogue feature was the cause of a program crash, an application compatibility problem, or worse, a security vulnerability.