These predictions were observed to hold most of the time, but in some cases such as the orbit of Mercury there were observations that contradicted predictions based on Newton's laws.
This led to their modification and replacement by relativity, which, rather than being a complete rejection of the Newton's laws, was a clarification and refinement that allowed them to hold true in a greater range of observable circumstances.
The Duhem-Quine Thesis is often contrasted with or considered to modify "naive" or "Popperian" falsificationism.
Imre Lakatos further extended Popperian falsification and the Duhem-Quine Thesis with his concept of "research programs." Lakatos defined extensively developed theories and techniques in a field as the "hard core" of a research program.
Because if you try to prove yourself wrong — and can't? The ability to evaluate theories against observations is essential to the scientific method, and as such, the falsifiability of theories is key to this and is the prime test for whether a proposition or theory can be described as scientific.
Then it's a really good indication that you're right. Put simply, if a theory cannot be falsified, there is no point in even examining the evidence. All scientific knowledge and theories are based on two things: observation and consistent logic.
Secondly, a discrepancy between theory and data does not necessarily falsify the theory.
For example, in the early 19th century, scientists discovered discrepancies between the orbit of Uranus as predicted by Newton's theory of gravity and the orbit which was actually observed.
If this question cannot be answered, then the conjecture is not scientific.
In addition, a good test of a theory is that it is able to make predictions about some future event.