What are some of the scientific sources of errors one can include in a lab? The errors can not be human errors.

There are many sources of error and that is why scientists
are so careful to control conditions as much as possible and why they repeat experiments
so many times before they come to any conclusions.

For
example, you may be timing some type of motion and only take two times and average
them.  If you had gathered more data you may have found that your initial numbers were
incorrect. Or you and another person are both timing the same event. One of the timers
may be faulty and so you get two different
readings.

Thermometers can be off by varying degrees and so
two different thermometers may give two different answers. It may be that one is correct
and the other not or that both are incorrect. In either case your reading is
incorrect.

When using a balance it may not be sensitive
enough for the experiment you are doing and it may need to be
recalibrated.

Chemicals used in lab experiments often
become contaminated because students put their extra materials back into what they think
is the right container but is actually the wrong
one.

Experiments are sometimes done in plastic baggies and
gases produced may escape through holes in the bag, throwing off a material balance  if
trying to prove conservation of mass in a chemical reaction.