Just adding to what Xyz( hope u don't mind)says:

It could be the case that all the session and hidden 
variables are not getting correlated, because of which the 
request you send with outdated variables(session / hidden), 
are found invalid by server.

Still this is a wild guess. Though, we can ponder more on 
it if you can provide more details about 
scenario<technology-platform of app and web-erver, script 
that got recorded while recording the action etc.>