The AAVSO has a policy in place that states positive observations having errors equal to or greater than 1.0 magnitudes are not accepted as is. For a given measurement, an error of 1.0 magnitude is equivalent to a signal-to-noise ratio of 1, meaning that the purported signal is indistinguishable from the average noise fluctuation in the data and statistically is not a significant detection. In practice, your software might report a slight increase in flux from within your measurement aperture, and your mind is always tricked into thinking that this is real rather than a chance coincidence since you know that the star should be there.
While the AAVSO values all observations by our observers, sound science is paramount, and from a rigorously statistical standpoint an observation with a signal to noise of 1 is not valid. In such cases, we strongly encourage you to find the faintest comparison star within the field having a photometric error less than 0.3 magnitudes, and report the observation as "fainter than (comparison star)". A fainter-than measure is a perfectly valid observation, and in this case is actually much more useful, because it puts a firm upper limit on the brightness of the variable at that time.
If you have questions or concerns about this policy, please contact AAVSO headquarters via email at firstname.lastname@example.org .