Comment #4 on issue 2116 by asmeurer: -LambertW(-1) is not real http://code.google.com/p/sympy/issues/detail?id=2116
So it's likely that you have found two bugs: one with is_real and one with the cache. I just point this out because fixing the first one might mask the second one by making it no longer happen for LambertW(-1).is_real, but whatever the real problem is would still be there (or at least that's a likely scenario).
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