Sitting wherever, if the phone drops the wi-fi connection it will switch to the cell network without warning or notice.
It's not until it tells you you've used minutes that you know.
Sure, usually just a few minutes, but some pages can cost 15-30 minutes in just seconds.
On the S390G you can use airplane mode to disable the cellular radios and still use wi-fi. Then if you loose wi-fi it can't take minutes.
The LG840G turns off airplane mode and turns the cellular radios back on when you use wi-fi.