These errors are usually reported in the following format:

Invalid read of size <bytes>
at 0x<memory address>: (location)
by 0x<memory address>: (location)
by 0x<memory address>: (location)
Address 0x<memory address> <error description>
  

The first line in the preceding error message tells one whether it was an invalid read or write access. The next few lines will be a back trace detailing the location (and possibly, the line in the source file) from which the invalid read or write was performed, and from where that code was called.

Finally, the last line will detail the type of illegal access that occurred, such as the reading of an already freed block of memory.

This type of error is indicative of writing into or reading from a section of memory which one should not have access to. This can happen because one accesses a wild pointer (that is, referencing a random memory address), or due to an earlier issue in the code which caused a wrong memory address to be calculated, or a memory boundary not being respected, and reading past the bounds of an array or similar.

Usually, when this type of error is reported, it should be taken highly seriously, as it indicates a fundamental issue which can lead not only to data corruption and crashes, but also to bugs which can be exploited by others.