A variable which is initially defined with += will end up with the variable undefined, or missing the initial definition. This is most likely to happen in complex buildouts with multiple overrides.

Such a case will corrupt .installed.cfg by adding an unparsable line "var + = base0" to the end of the offending part:

[base1]
__buildout_installed__ = 
__buildout_signature__ = wheel-339de3cbbb8d2a18c6d8133fb43aa777 zc.buildout-1a1dfc3477a86238088ba01adcf15436 pip-339de3cbbb8d2a18c6d8133fb43aa777 setuptools-339de3cbbb8d2a18c6d8133fb43aa777
recipe = zc.buildout:debug
var + = base0