What is Acceptability Testing?

An acceptability test is one where a set of inputs is run through the System Under Test (SUT) and checked to make sure the correct outputs are produced.

What is a Bug?

An execution error
Incorrect output

What are the possible sources of bugs?

The SUT
The Acceptability Test
The Specification
The System in which the software is run

How do you find the source of a bug?

Check the possible sources of bugs in order.

SUT: syntax, logic, etc.
Acceptability Test: same as SUT plus the input set
Specification: Understanding of what the SUT is supposed to do
System: Unfixable, look for workarounds and ways to report the problem