There are many possible causes for this error. Using MiniVend 3.08 and above, the main 3 are:
This most common case is where the user bookmarks a shopping cart after Recalc or bookmarks a checkout page. Those pages are usually the results of form submissions, and are not really a candidate for a bookmark. You should make sure you provide links to both of those pages on the interact special page (in special/interact if you have started with the demo).
User hits BACK with a very small cache. There are quite a few users who have decided 10Kbytes is enough of a cache, and that won't quite cut it. 8-) (Actually, they probably assumed the number to enter was in megabytes.)
One possible cause is that you must define METHOD=POST in all of your forms, not METHOD=post. The lower case form is not recognized in the HTTP spec.
If you are using SSL and don't have a later HTTP server (released after say September of 1997), there is a bug in earlier versions Microsoft Internet Explorer's version of SSL3. Stronghold 2.1.1 works around this bug, so if you use Stronghold you should update.
MiniVend also allows you to have forms with METHOD=GET. To use, set the session ID in your form
<INPUT TYPE=hidden NAME=mv_session_id VALUE="[data session id]">
You might then try METHOD=GET on forms that have problems. This may prevent errors due to one of the causes above.