[This is very old news to old timers, but I was spoiled at my last job and had mains-powered DMMs for day to day use. So, this is news to me...]
From the "I know better, but I'm too lazy to crack open my meter and do the right thing now department..."
I have a Fluke 79III that was given to me by a coworker a few years back. It works fine - or at least I thought I knew how it worked. It calibrates out perfectly. I've been using it strapped to my pegboard as a surrogate bench meter for when I'm too lazy to boot up my real bench meter.
A couple of days ago, I was getting all sorts of crazy readings off a voltage regulator. I assumed that I had screwed up the regulator somehow. The battery indicator on the Fluke was on, so I figured that it meant: "I will die soon, but will give you good readings for a little while longer." Nope.
What that indicator means is "my battery is at 6V, very very soon I'm going to give you complete shit readings until my display fades out completely." With the battery flag on, that meter will display readings up to 2X of their actual value. For the 79III, Vbat=5V is when the readings go south. I honestly expected the meter to have internal logic that would shut it off if it didn't have enough battery voltage to give correct readings. Not so.
If you have a DMM and a variable power supply, it might be worth your time knowing what it means when the battery low flag comes on. This isn't meant to put down Fluke at all. They did tell me to change my battery. And I didn't. I simply assumed that the meter had more smarts than I did.