After automatic PSD to HTML conversion it was possible to get incorrect XHTML/CSS layout in case of using $bg$ tag in a not right way. This error in the PSD to CSS online conversion was fixed. Below is a description in what case you could got this error.
The first thing you should understand is that only the lowest layer in a layer group can be set as a background for the group. Otherwise, z-order of layers will be corrupted.
Formerly, if user marked not the lowest Photoshop layer with a $bg$ tag, it would have been converted into a CSS background anyway. Thus, in some cases this layer might have been overlaid by another layer and become invisible. So, PSD to HTML conversion might have been inaccurate.
Now, this is not a case. Psd to CSS online service will check z-order of layers and will not convert a layer marked with a $bg$ tag into CSS background in case in will cause incorrect z-order of layers. Instead of it, service will automatically find a layer that is best to become a background for the layers' group. Thus, design to HTML conversion will always be accurate.