I would imagine that this is a bit of an anomaly and I hope that it does not happen anywhere else but:
I've gotten my aluminum tank filled twice at the local beer distributor and both times I had to refill within 2-3 kegs (all 1/4's of commercial). I chalked it up to a leak in my system but after inspection I didnt see any bubbles using the soapy water method. So I went back and talked to the guys to see what they use to refill (these guys are a little shady, but the only place that "fills" CO2 around me.
Turns out they dont fill the tanks correctly. I weighed my tank when I got home from the third fill and it was less than 1 lb more than the tare weight. So I asked them, 'how do you fill your tanks, do you weigh them to make sure its 5 lbs?'.
"No, we have a 100 lb tank that we hook it up to. Since your tank only holds 5 lbs, it will pressurize until its full."
"Ok, that makes sense, is your 100 lb tank's output on the bottom, so that it is filling liquid"
"No, it doesnt need to be. CO2 is CO2."
I tried to explain to him that if you are only pressing gas into the the smaller tank, CO2 will stop when the pressure equalizes, not guaranteeing that the 5 lb tank will be full. But they didnt want to hear it, they told me that that's how theyve always done it and have never had a problem.
So I put down the case of yuengling I was going to buy and walked out. I will not be patronizing this distributor anytime soon.
I called around and talked to another distributor that has always been a go to for some of the harder to find craft beers (about 25 minutes away from my work). They said that they fill tanks and ensure that they will be full. I'm gonna check them out when my current 1 lb of CO2 runs out.