Looking at XOM (but it is the same for many stocks) there is an odd drop in the stock price during 1988. It drops around the new year and pops back up around the next new year. This drop is present in a lot of stocks reported by google finance. The same shift is not present on Yahoo Finance and as far as I can tell it doesn't coincide with actual market data. So is this just a glitch? Do others see this?
More specifically: is there an explanation for this error? Why 1988 across so many securities? Is it an algorithm oops, or was there some underlying change in reporting or source data that could cause it?