Could certainly be a bug. Are you fully patched to CS5.5 7.5.2?
Which version of ADE are you using? I suggest downloading the 1.8 preview version from labs.
Finally, I would check this on a tablet to see if it’s happening there. EPUBs can easily be sideloaded onto an iPad for viewing in iBooks.
Thanks Bob for your answer.
The version is ID is CS5.5 - 7.5.2, the last one.
When I look the code of the toc.ncx generated in the epub, the problem is there :
On some readerlike Ipad, the display is ok, but it can be a potential problem for most of the readers.
I'll try to report this as a bug.
In that case, it looks you have a bug on your hands.
You can report it here: https://www.adobe.com/cfusion/mmform/index.cfm?name=wishform
You can also check this little utility out: http://code.google.com/p/epubfixer/
I won’t say I’ve done any thorough testing on it but I did play with it a bit and it was very helpful in fixing up TOCs.