Mobile carriers are going in on 'unlimited' data plans, but they're all full of red flags

It's about that time of year where all the major US carriers shake up their mobile plans.

This year, the buzzword is "unlimited." Both T-Mobile and Sprint on Thursday introduced new plans touting unlimited talk, text, and data. On Wednesday, AT&T refreshed its offerings, loosening its policy on overage fees in the process. Verizon made similar changes in July, too.

Per usual, though, the big four have largely obfuscated what exactly their "unlimited" plans are doing. So let's break things down.