You tell me when it happens.
As for why, here's a quote from the Online Manual:
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Warning W1301: Can't open file
The given file was not found in the path or resource.
Error E1302: Bad MDL format
The given model file was damaged or has a wrong format.
Warning W1305: Bad File Name
This warning is only issued when the warn_level variable is set to 2 or above. The entity file name contains invalid characters such as ' ', '/' or '\' - the latter is interpreted as a path and will lead to publish errors.
...
As you can see, malfunction W1303 isn't listed, but we can see that if it was, it would be grouped among the "file not found" type errors. ie your script is probably looking for a file that is either damaged or does not exist. It could be a sound, level, model, or even a text file.
Hope that helps
