These are examples of barcodes that could potentially cause problems
I'm sure will be able to find examples of barcodes similar to those below that do scan. You may even use them yourself. But remember, if you have a high quality (often expensive) barcode reader the manufacturer may have built in features to compensate and reduce the percentage of fail scans.

Colour combination won't scan (Readers use red light!)		Background colour prevents scan		The same GTIN is on different product lines		No edge - scanner can't read the code
Barcode height is below GS1 recommendations		Barcode height is below GS1 recommendations		Light margin at the edge of the code is not wide enough		Light margin at the edge of the code is not wide enough
Reflections of metal background or bars can cause problems		Barcode is obscured		Packaging barcode obscured by wrapping		Barcode is obscured by wrapping
Printing barcode directly onto packaging distorted code		Packaging wrap obscures barcode		Barcode has peeled and crumpled		Barcode is peeling and obscured
Printing barcode directly onto packaging distorted code		Barcode printed out of proportion / Ink has bled		Product and packaging barcode can be seen		Product and packaging barcode can be seen
Very small printed barcode		Barcode height could be difficult to scan		Faulty print - barcode might not scan		Faulty print - barcode might not scan
Barcode is printed vertically		Poor material quality and ink has been absorbed and bled		No background colour to the barcode - product contents transparent		No background colour to the barcode - product contents transparent
Barcode has wrapped around the product		Barcode has wrapped around the edge of the packaging
Images, courtesy of GS1 UK