The dangler is that little blob that gets left dangling when making a schematic connection and you double click in free space. It is designed to allow you to temporarily leave a connection dangling while you sort out something for it to connect to.
Simply put - Danglers are created when you terminate a connection in space.
I.E. you bring a connection from an IC in one corner of your schematic sheet to the other corner only to discover that you have forgotten to add the part it connects to, you double click and drop a dangler leaving the connection there - saving having to redraw it all while you add your part then continue adding the connection from the dangler.
Apart from junctions, signal references and global signals, a dangler is the only way to leave a connection without it being connected to a symbol terminal.
So what is the warning?
Well, if you are drawing your schematic on a very fine grid, (I.E. 1th) you have to be extremely accurate connecting to symbol terminals or other connections, if you miss and inadvertently double click then you can leave a dangler instead.
Unfortunately if your dangler is the same shape as a junction then it looks the same and you do not notice that the connections are not actually the same net.
|Here the vertical connection has been|
pulled to the right to show the dangler.