soft limit was 20Gb
"they won't even tell you what their secret limit actually is"
It used to be 20Gb to trigger a 'what are you doing' message. Give the right answer and nothing else happened, ignore it or say 'I'm tethering' and data got terminated.
Now they seem to be a bit more aggressive, actually trying to guess if you've been tethering and cutting off some at the time that question gets asked. Again, answer right and you're reconnected. Don't know if the 20Gb still applies but managed about 3.5Gb over Xmas without any problems.
A big problem is a widely accepted belief where 'bargain hunters' gather that tethering is included in the unlimited offer, a meme that never seems to die. Probably because the ones that do know it's forbidden aren't slow to state they'll do it anyway and don't believe they'll be caught! Every time giffgaff get's mentioned on HUKD you see it and everywhere else I've stumbled across.
It's why I don't pay much attention to whiners complaining they've been unfairly cut off, odds are they've been completely fairly dumped after getting away with it for too long. There must be false positives, giffgaff are pretty incompetent at anything technical, I just don't believe there are many.