This last week, meanwhile has been a hectic rush as I was initially blamed for submitting broken tags into Athena. It turned out it was a subtle bug in the pixel software which went something like this:
Gaudi has a templated stream insertion operator for all types.
A PixelDcsData class also had a stream insertion operator.
A PixelDcsData class also had a single argument constructor, that argument being 'Identifier'.
Now, when trying to output 'Identifier' with stream insertion, the compiler complained of an ambiguously overloaded '<<' because it couldn't decide whether to convert Identifier to a PixelDcsData class and then use the custom '<<' or use the intrinsic templated '<<'. Constructors with a single argument act as conversion operators. To avoid this, qualify the constructor with 'explicit'.