Error deleting model dll

Sometimes when trying to save a model an error message will appear, similar to,

Simile encountered an unexpectected  problem: error deleting modelpath/model.dll permission denied

This happens when

1. a model that had already been built was loaded (and so an executable model had been saved with the model specification), 
2. a change was made to the model
3. and then the modeller tried to save  the model without rebuilding an executable (i.e. selecting run on the model window Model menu).

This not a severe error, the model specification will be saved. When the model  is next loaded the executable can be rebuilt from the specification. (Model executable is only saved as a convenience to save rebuilding each time the model specification is loaded.)

Work around:

Rebuild the model by selecting run on the model window Model menu before saving the model.