It probably wasn't child porn at all, but mislabelled as such. You failed to mention which
program you were using but LimeWire is known to have a weakness where a download can be
renamed before it completes if a large number of the sources have a different name for the file.

From now on check your search results with bitzi before you start them. The command is
hidden under "Advanced" when you right-click on the search result.