any thing that might affect the outcome of an experiment?

Answer 1

The question is too general. The depends on the objectives of your experiment. Could be the temperature, the humidity, the pressure, among many others. Could be any factor that independently or dependently may affect whatever the variable or effect that you are trying to measure.
For example if you want to measure the acceleration due to gravity, it may be affected by: the friction with the air, so you should design your experiment trying to avoid that effect, or do experiments to try to quantify that effect. So, experiment under vacuum, objects with different shapes, with different mediums, at different distances, etc.
In order to compare your results, you need to be extremely systematic, so much so that everybody can reproduce your experiments. If the same results are obtained elsewhere, it means that you don't have something that can affect the outcome of your experiment.

Answer 2

When you design an experiment, the whole point is to remove anything other than what you are trying to observe that might affect the experiment. In addition to isolating/controlling physical conditions such as contamination, temperature, and time, you need to isolate/control human intervention. Quite often, humans accidently or intentionally coax an experiment to give wrong data. Testing medicines come to mind. This is why a well designed experiment will use a double blind method, in which the experimenter is eliminated as a source of error. This is why you hear reference to "placebos" on commercials for medical products. Failure to do this is also the classic reason for incorrect conclusions by experiments for ESP and other paranormal claims. Inevitably, EVERY test for paranormal ability fails when evaluated with double blind testing.

In scientific circle, an experiment is considered valid if it is repeatable. This "rule" for repeatability helps other scientist determine if all other factors that might lead to the experiments conclusion have been eliminated.

Answer 3

How you measure the result of the experiment can have a big effect on the outcome. Actually, everytime you measure something, you alter the thing you are trying to measure. For example, measuring the air pressure in a tire can involve letting out some air, which changes the very thing you're trying to measure. So carefully consider how you are going to measure the outcome.

Answer 4

Knocking the apparatus off the table will usually have an effect.

Answer 5

time, light, temperature, pH, contaminations, the list goes on...