All of our data loggers automatically insert error detection codes into the log memory while they record. When these detection codes for a block of recordings fail the integrity validation, the relevant readings are marked as non validated readings. On your data list, non validated readings are displayed differently to validated readings by default, to indicate that they have failed this integrity test. For the chart, this option can be enabled in the option settings. These error detection codes are saved with the file so any attempts to manipulate data in a saved file are also detected.
There are several reasons why data can be marked as non validated:
Only in very rare cases is this a result of a memory fault in the logger itself.