Every so often, I try to look at the source code for some function. It often doesn't work. There is a commonality to the times where it doesn't work. The source code usually has examples in it. And, often there are examples of things that don't work, that cause errors, where the error messages are shown in the source code. It is these examples that seem to cause the problems. The source code prints out up to the error message and then it just stops.
The latest source code I tried to view is:
sage.graphs.graph_coloring.all_graph_colorings??
The last thing I see in the source code is:
sage: for C in all_graph_colorings(G,-1): print C
Traceback (click to the left of this block for traceback)
...
I am using Firefox 11 to access Sage on a server, in case that matters. It has been happening for a while, so also it has applied in some previous versions of Firefox as well.