Just as the title says...has anything like this happened to anyone?...
A few months ago, I deactivated one of my lines on a non-Sprint MVNO (I'm not going to name it in case someone from the carrier is reading this). I had intended to port out the line but it wasn't important enough to keep and I wound up deciding to just tell the carrier to deactivate the line. That brought me down to 4 lines on this carrier, although I have since added back a line on a different number with a preferred area code.
Last month I was playing around with a very old model phone in which I had stuck the SIM card associated with that presumably deactivated line. I was playing around with just wifi for a bit and then wound up getting a call from a junk telemarketer. Lo and behold when I tried making an outgoing call, the old number still worked. Not only that, I was still getting LTE data. I doubled check my account online and it had no record of the number being in active status.
For the next three weeks, I swapped the SIM several times onto different phones and used it mainly to stream Pandora/Spotify and read various news publications, racking up about 3 GB. My billing cycle just ended and I was not charged for any of that use.