In the design of programming language

Programming language

A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....

s, an erroneous program is one whose semantics are not well-defined, but where the language implementation is not obligated to signal an error either at compile or at execution time. For example, in Ada

Ada (programming language)

Ada is a structured, statically typed, imperative, wide-spectrum, and object-oriented high-level computer programming language, extended from Pascal and other languages...

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In addition to bounded errors, the language rules define certain kinds of errors as leading to erroneous execution. Like bounded errors, the implementation need not detect such errors either prior to or during run time. Unlike bounded errors, there is no language-specified bound on the possible effect of erroneous execution; the effect is in general not predictable.

Defining a condition as "erroneous" means that the language implementation need not perform a potentially expensive check (e.g. that a global variable refers to the same object

Aliasing (computing)

In computing, aliasing describes a situation in which a data location in memory can be accessed through different symbolic names in the program. Thus, modifying the data through one name implicitly modifies the values associated to all aliased names, which may not be expected by the programmer...

as a subroutine parameter) but may nonetheless depend on a condition being true in defining the semantics of the program.