It can not occur
accidentally

answer why


It's a bug in the Bitcoin software, where certain transactions are
considered valid, even though they aren't. Fortunately they bug only
affects part of the code and these transactions won't be included into a
block.
So until that bug is fixed, the Bitcoin monitor might sometimes show
strange transactions. They won't be included in a block though, so it's
not a big problem.

Regards,
Jan

Is this Fraud or error
this is a screenshot
by http://www.bitcoinmonitor.com/
10 44 42-29 07 2011
https://picasaweb.google.com/103217016766833872478/292011#5634833875507002066