Open up a daily chart with SPY going back a year. Look at 8/31/11. stockcharts.com shows the high as $121.12. Yahoo finance also shows the high as $123.51. freestockcharts.com also shows the high as $123.51. edit - I uploaded the wrong image, hang on
If you go back through the data of any of the feeds... TDA, ThinkOrSwim, FreeStockCharts, OptionsHouse, etc etc etc... You will find errors here and there... In all of them. There are a few professional quality feeds offered from places like DTN and etc... But for clean quality feeds your going to be paying $200-$5000/month. You get what you pay for! Personally... Perfect feed quality doesn't matter to me unless it's intraday futures trading... Then a clean solid feed is definitely necessary. If your doing very short term trading in time spans of less than a few minutes investing in a high quality feed is necessary if your futures broker doesn't have a high quality enough one... Otherwise who cares if prices are off a little here and there.
Oh, and as another note... It's bad practice to chart a derivative... The SPY ETF is derived from the S&P 500... It's an instrument that algo's keep in sync with the SPX and EMinis if prices happen to be out of sync by enough to make a possible profit by moving in to bid up mispricings in milliseconds. In addition alogo's also play the mispricings between SPY and it's underlying stocks helping create a slight amount of decay in the ETF. Should be illegal to game an ETF vs it's underlying stocks but thats the way it is... These ETF's always fall prey to the algo's during rebalancing. Just chart the SPX(Or Symbol INX). Nobody bothers to chart the spy... In addition the SPY is well known for having zillions of bad ticks.
I noticed descrepancies like this a while ago. All the data around the 8/31 you mention is off by similar amounts. One guess is it might have something to do with how each data vendor deals with the ex-div date price adjustment/reduction. For instance look at June 15. That "should" have been a gap up, and stockcharts.com shows it as a gap up, but TOS shows the ex-div adjusted data so there is no gap up and instead June 15 opens right around the prior days close. And yeah SPY does have it's share of bad ticks and some data vendors never corrrect them, so that could be part of the problem too. A while back I compared Thinkorswim, Prophet, stockcharts, freestockcharts and there were definitely days where SPY didn't match up across the four. But from the stuff I looked at $SPX always matched up to the penny on highs and lows so I'd say that's more reliable, even though it sucks that it doesn't show the gaps and always shows the open = prior days close. EDIT - what's interesting is although TOS and stockcharts.com show different price action on the 6/15 ex-div date, the highs/lows match up since then. But PRIOR to the 6/15 date, sc high/lows don't match TOS. So it looks like sc is adjusting high/low data prior to the ex-div date, to account for the fact that they don't show the ex-div price reduction. Maybe this is the reason? But there are a lot of other smaller descrepancies if you look at enough data, probably due to bad ticks. So to be safe I stick with $SPX as my primary source for S&P levels but still look at SPY to get a better feel for the gaps, etc