We were getting the following error quite frequently in CiviCRM.log:
"We could not find contribution details for your request. Please try your request again."
From the backtrace I found that the message was apparently being generated on line #134 of ./CRM/Contribute/Form/ContributionBase.php. I dumped $_SESSION and $_REQUEST and found that while the code was referencing the session variable as "pastContributionId" that the actual session variable was named "pastContributionID," with a capital D. Changed this and the errors went away. Also did a recursive grep of the entire CiviCRM codebase to make sure that it was referenced everywhere else with a capital D, and it appears to be.
The attached trivial patch should fix the problem.
We are running v2.0.3