[rinnytinny]

No data is lost... if you're doing it wrong and hit backspace, hit 'page forward' to correct your blatant error.

[Joakal]

On Firefox with Google's main search boxes featuring autocomplete=off; the input gets erased every time.

[pbreit]

I lose data all the time. Maybe I'm stupid.

[StavrosK]

That depends entirely on the browser.