This is a potential bug in Visual Editor
Imagine if you have this:
- A definition list (dl) term (dt)
but without a definition (dd).
Now try and insert a header in before the dt. Doing so can make it very easy to remove the dt accidentally.
The dt looks just like bolded text rather than a dt. Definitions are used non-semantically in Wikipedia a lot. (Of course, one solution to this would be to go around ruthlessly removing dl/dt/dd combos that aren't actually doing any defining.