Let's assume we have a layer in a doc at a default opacity of 100%. There is a problem that can occur if you set the value of the opacity to 100 via a variable. It works ok if you set the value to 100 via a literal constant:
Code: Select allvar doc = app.activeDocument;
var layer = doc.activeLayer;
// displays "100"
alert(layer.opacity);
// Error: Specified value greater than maximum allowed value
layer.opacity = layer.opacity;
// OK
layer.opacity = layer.opacity.toFixed();
// OK
layer.opacity = 100;