This is very old code! The segments are stored as a linked list and insert_segment just inserts a new segment into the list - should be ok but if one of the segments is being deleted then the linked list would be broken and the observed behaviour might be seen.
Can you reproduce this in a tcl script rather than via the labeller, that would give us a better chance of isolating the bug...
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This is typically to do with problems in the hlb file where
the label file (.lab) and hierarchical label file (.hlb)
have become out of sync. Try deleting the hlb file and see
if the problem resolves itself. It could be though that a
time of -1 has been recorded for a segment in your .lab
file, in which case Emu will always give the above message
(-1 is...
