I recently ran into this problem while installing 4.4-RELEASE into a
pre-existing slice.  It is discussed in -questions with the subject
"Partition table problem".  The exact details are impossible to know due
to data loss, but it appears that I somehow had two slices overlapped
by about 600MB (how this happened is beyond me).  The "partition editor"
appears to have detected this error and zeroed out the partition table
entry it decided was in error, and sysinstall then wrote the change to
the partition table.  I did not actually make any changes in the
"partition editor".  I entered it only long enough to remember that
I had already created the partition at an earlier date, then quickly
exited and went on to the label editor.  Obviously the actual data
loss occurred when the installation overwrote the data in the overlapped
slice, and was not the fault of FreeBSD.  But data loss could have been
avoided if the "partition editor" had popped up an alert about the
invalid slice configuration and requested confirmation to correct it,
rather than doing so silently.