There seems to be an issue with TClass::IgnoreTObjectStreamer for ROOT 5.34/20 and ROOT 6 checked out from GIT today. With ROOT 5.34/19 everything works as expected.
The attached code creates a tree to store a simple class deriving from TObject only holding a single double. The information of the TObject itself should not be stored, so I call IgnoreTObjectStreamer for my simple class.
In the case of 5.34/19 the resulting tree looks as expected, there is a single element belonging to the class. With 6 I instead find two elements coming from the TObject (fUniqueId and fBits) associated to the double, with 5.34/20 the TBrowser crashes when clicking on the "number" leaf (however, if more than one double is stored, e.g. if number is replaced by a vector of doubles, the result is the same as with ROOT6).