Testing Kits - A Cautionary Tale

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.

I doubt an unseen residue can give a reading of 140. I’d say it’s more likely that your nitrate kit is on the way out. Do you bang it on the table before use?

I rinse my vials twice. Whatever is left after that isn’t enough to affect a reading.

Shaking isn’t enough. You need to literally bang it on a hard surface to dislodge the flakey solid deposit on the bottom. Otherwise after a while you’ll get false readings.

It’s the #2 reagent of all of them. One of the ingredients doesn’t like being in there, and precipitates out as a solid on the internal surface of the bottle. It needs to be dislodged, then shaken.

I should have done a bit of research shouldn’t I? I tend not to use a nitrate kit these days, and haven’t looked into them for a long time. I think it would just be accurate to say that any that need to be shaken would also benefit from being banged against an immovable object a few times.

Thanks, Sue - that's very helpful indeed.
On the basis of that, I decided to have a look to see what the ingredients were for Waterlife's nitrate kit (powder plus 3 liquid bottles) which I've previously used, but their website isn't functioning.
Anyway, this whole discussion (thanks, both) has helpfully confirmed for me that:
* I'll be more thorough in my cleaning regime, esp of the vials' lids, and in my drying regime (proactive with kitchen paper rather than passive air-drying);
* I'll continue with nitrate tests that have the sulphanilamide/sulphanilic acid in powder form, as shaking a bottle to the extent required is not feasible for me and banging a bottle off a table more than likely to result in a nasty incident or domino effect of incidents given my accident-prone tendencies from the most innocuous of activities.