Ordinarily, I carefully fill, swill and empty each vial 3-5 times with a strong flow from the tap, then approx 5 times on a weaker flow, then leave the vials out to dry. I now have 3 test kits of the same brand, and admittedly the vials for each have not been kept with the respective test kit. I never thought anything of this until yesterday.
Since the merger of main and quarantine tank inhabitants, I'm doing daily water tests for the foreseeable future just to ensure that all is well and the merged media is doing as it should. Imagine my horror when, although ammonia and nitrite were clearly at the lowest possible reading, the nitrate level was not at its usual second-lowest or somewhere between second- and third-lowest reading (1-5ppm) but 140ppm! Horrified, I left the vial to the side and used another - 20ppm. I did the same for a third vial - 1ppm. Three different readings for the same sample of water. [I couldn't even cross-check with the nitrite/nitrate strips as they're the strangest I've ever come across for nitrates (a poor and much-regretted impulse purchase), giving a dark grey shade instead of somewhere on a scale of light grey through various shades of pink to dark pink.]
Thankfully, all was fine later on and again this morning, with 0 for two tests of tap water and 1-5 (closer to 1, for sure) for three tests of tank water. At this rate of multiple tests, I'm going to have my test kit used up very swiftly!
I can only assume that, somewhere along the line, the seemingly dry vials had the residue of a previous test, possibly even for ammonia or nitrite in it, accounting for the bizarre readings.