When cookies are disabled while using sessions+transsid, php adds a hidden field directly behind the form tag.
This has influence on the document-model of the created site: since all elements of the form can be adressed via javascript using an array (e.g. forms[0].elements[0]), the newly created hidden field shifts all members of the array by one point.
This can cause off-by-one errors when accessing the array with direct addresses, resulting in strange behaviours on the client (wrong line of data used etc) - only with cookies disabled.

Of course, there are other possibilities in adressing the elements, but there are some solutions that would ease the problem and stop php preventing the user from using one method of adressing the elements:
- put the hidden field right before the end-form tag (bad solution, object model is still affected, but additional elements at the end obviously cause less problems...)
- don't put the hidden field if the user manually uses SID in the form, e.g. in the action of the formtag (better solution, enabels advanced users to control how the object model is affected)

i think a combination of both methods would increase some inexperienced users sites (that never test their site without cookies) and experienced users can control the functionality.