You learn something new every day
I’ve been writing C# code since one of the early betas back in 2001, yet I’ve never run across the following:
error CS1638: 'ToString_WithCache__ShouldReturnICompositionElementDisplayName' is a reserved identifier and cannot be used when ISO language version mode is used.
I was weirded out by this and thought ‘that’s a hell of a coincidence that the C# compiler has an identifier called ‘ToString_WithCache__ShouldReturnICompositionElementDisplayName’, but as it turns out identifiers with two consecutive underscore’s are actually reserved:
Identifiers containing two consecutive underscore characters (U+005F) are reserved for use by the implementation; however, no diagnostic is required if such an identifier is defined. [Note: For example, an implementation might provide extended keywords that begin with two underscores. end note]
You learn something new (however useless) everyday.
(To see this warning you’ll need to turn on ISO-1 in the Language Version drop down in the Advanced build settings)