How old is the bird you suspect laid it, and what is she currently being fed?
If she is a pullet still on Grower's feed I would recommend that you put out some oyster shell if you haven't already, and as soon as she lays a 'normal' egg, switch her over to layer's pellets if she's not on them already.
Soft shelled and tiny eggs are normal when a pullet first comes into lay so I wouldn't be concerned just yet. However, if this is from a hen who has been laying for a while it might be that she needs some additional calcium support.
If it's an occasional thing only I wouldn't be concerned, but if it becomes regular maybe you could try a calcium supplement for a while.
Good luck,
- Krista