Example of clogged needle due to polimerization of protein as it is reacts with the media.

This is a common problem with DNA (as it exits the needle it polymerizes as it is forced through the constrained glass tip) or with hydrophobic proteins as they enter the media.

The cone at the left is polymerized protein.   The glob at the tip of the needle is polymerized protein.  When this problem occurs with DNA, strings or stands appear to be hanging out of the tip of the needle.