'The submitted form was invalid. Try submitting again.'

[Alex]

I keep getting the following message:

The submitted form was invalid. Try submitting again.

When I try to post things, but I'm not sure why it's "invalid". Sometimes I have to change the wording of a sentence, but it's a royal pain to do when I haven't posted anything wrong o.O Does anyone know the cause of this error, or how I can avoid it?

[Mongrel]

I get that sometimes if I've left the page open too long. My internet kicks off and on all the time, and I think that's what does it.