Bug Calibre editor

[Stevex]

This is an incredibly rare event so I post it 'for the record'
Using Calibre 3.0 on a fairly slow 32 bit windows 10 PC
Sequence of events
Using book editor on epub
Cpu and disk very busy because just plugged iphone in and itunes was syncing, we are talking about 60 second plus response times
Added <span class="italic"> into a para. Preview window shows text italicised till end of para.
Added </span> after word to italicise. Preview window didnt change. Click around, preview window doesnt update. (This is nothing new has happened to me before with </span> when pc busy)
Clicked in another tab of another html file in the book. Waited to see that page.
Clicked back on the tab of the page I had been editing.
Wow!!! Both the <span class="italic"> and </span> had disappeared!!! Like rolling back a database transaction.
Well... i was using Calibre a lot for years and never saw anything like that before. Obviously it's some kind of one in a million fluke, but still worrying that it can happen.

Ps by the way the new qt in cal3 seems to have fixed the disappearing characters in the editor