When I loaded a Scratch 1.4 project which contained list contains (), into Scratch 1.3 it turned into an if obsolete block.
I ran the script, an error occured!
I shift clicked R to turn off error catching, and I got an error: NonBoolean receiver-proceed for truth.

This seems to be a Squeak error, as a block boolean* will return false in Scratch 2.0 & over.

*a block boolean is:

if (broadcast [ v] :: stack) then
end

This is because there is no such thing as

<obsolete!>

, the obsolete blocks can only be stack blocks.

This returns an error because stack blocks do not return anything. You aren't really supposed to load projects from newer versions into older versions, so this is not unexpected.