Description of problem:
By using this code:

digraph d {
	edge [constraint=false];
	a -> b;
	b -> a [label="whatever"];
}

Version-Release number of selected component:
graphviz-2.28.0-26.fc18

Additional info:
backtrace_rating: 4
cmdline:        dot -Tsvg -O twitter.dot
crash_function: makeSimpleFlatLabels
executable:     /usr/bin/dot
kernel:         3.7.2-204.fc18.x86_64
uid:            1000

Truncated backtrace:
Thread no. 1 (7 frames)
 #0 makeSimpleFlatLabels at dotsplines.c:910
 #1 make_flat_adj_edges at dotsplines.c:1117
 #2 make_flat_edge at dotsplines.c:1457
 #3 _dot_splines at dotsplines.c:440
 #4 dot_splines at dotsplines.c:504
 #5 out at dotinit.c:275
 #6 gvLayoutJobs at gvlayout.c:90