Was it a Sensor Error #1?
We've had a few of these lately and all we can attribute them to is when we've calibrated after being out of range and there's a big disparity between the numbers.
We usually leave it, and then it'll go to ??? and usually rights itself after an hour or two. Although last weekend we had to replace 2 sensors on consecutive days (Dexcom are ALWAYS awesome about sending replacements, so always call them in!).
What I have started doing now, with better success, is "shut down" when he's going swimming (or out of range), then turning it back on when he's finished. This has virtually eliminated the problem.
Suzanne, mom to
Kieran - 7 yrs dx Oct 2009. CGM since 3/24/10, pumping with blue Animas Ping since 4/26/10
Ellie - 4 yrs non-D