Master and origin/master diverged

Hi all,

One of my developers managed to push something that somehow "diverged"
origin/master from everyone else's local master.

A --> B --> C --> D (everybody's local master)
|
\--> B' --> C' --> D' --> E (origin/master)

(i.e., A is the commit where things diverged; everyone's local master
points to D but the new commit (E) that was pushed to origin/master
uses different SHA1s for B, C, and D). I hope I explained it clearly
enough.

Now running git pull creates a merge commit joining D and E.

When I asked the developer what he had done he told me everything had
looked normal. No Git warnings, nothing strange in gitk (i.e. normal
linear history). So I do not understand what went wrong.

Does anyone have any idea as to what might have happened? Perhaps if I
understand how this happened I might be able to prevent it from
happening again.

Cheers,
Hilco