If harmfull (>V+, <V-): I'd clamp it (hard). I'd use transzorbs, zeners or other protection circuitry at the inputs/outputs anyway against ESD/EOS. If bizzare: I'd clamp it with zener or transistor/op-amp (precision) clamp. If just outside normal range: No action. Just change the way you look at it. Maybe something interesting comes out. m.c.