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Issue #648 seems that is caused by a savegame created before c9b33fc . This savegame is incompatible with newer versions of ResidualVM, since it contains an unpatched version of si.lua that expects a different behavior from the interpreter.
A possible solutions for this type of problems in the future, would be bump the savegame version if patches like that are developed.
Preferably the saves should know which patches (and perhaps even a version-number) were loaded, as there is no complette guarantee that this will not be overriden in some interesting way by the user (i.e. the user might simply compile from source, and deploy the patches in his game-folder, making for interestingly incompatible savegames).