I've actually seen a number of times where Google Finance has a daily low that was never reached or a high that wasn't shown on the graph (sometimes many percentage points away). It's also notoriously bad when there's a gap up or down, and will sometimes cut out various parts of the graph.

But still, seems to be one of the better ones out there, as I still use it. Just don't treat it as authority.

Another interesting example of questionable data is the premarket CNN feed at (http://money.cnn.com/data/premarket/). I have sometimes seen a stock trading premarket down 99%, which almost always looks like an off by some factor of ten of the price.