From the RSPB: Mesh bags – a warning
Peanuts and fat balls are regularly sold in nylon mesh bags. Never put out any food in mesh bags. These may trap birds’ feet and even cause broken or torn off feet and legs. Birds with a barbed tongue, eg woodpeckers, can become trapped by their beaks.
More info here:
The RSPB: Advice: How to feed birds
(I believe they have actually received reports of such things - including fatalities.)