The NOAA/NASA Pathfinder AVHRR Land (PAL) product is one of important data products for studying global vegetation. It contains global NDVI time series derived from NOAA-AVHRR at 8-km and10-day resolutions for the past two decades. Many studies have used the PAL product to analyze the global vegetation dynamics and trends in the past two decades with an assumption that either PAL has no systematic temporal errors or the errors are much smaller than the signals of the global vegetation changes. However, this study finds the significant systematic temporal errors in PAL due to the orbital drifting of NOAA POES satellites that increases the global NDVI value with aging of a satellite. Another source of NDVI errors is the volcanic aerosols, which have large negative effects on global NDVI value in the product. The aerosols brought into atmosphere by June 1991 eruption of Mt. Pinatubo reduced the means of the annual global maximum NDVI by 0.03, 0.067, and 0.027, for 1991, 1992, 1993, respectively. The aerosols from the eruption of El Chichon in 1982 reduced the mean of global maximum NDVI 0.016 NDVI unit for 1982 and 0.022 for 1983. Those errors are larger than signals of global vegetation change due to the climate change. After removing the errors by a statistics-based algorithm, it is found the mean of the annual global maximum NDVI increases at the rate less than 0.005 NDVI units per year.