Half of my MX29F800 chips fail to verify. I purchased 10 brand new from digikey and 6 of them fail to verify while 6 verified successfully. On every chip that fails: Chip ID is correct. Erase is successful. Blank check ok. Write is successful. Verify fails at different locations every time.
Upon reading the data back and using HxD to compare files, there are as many as 10 errors!
I also bought 5 MX29F400 eeproms and all of these worked perfectly.
Something odd I have noticed is that the Device read data that is greater than the buffer. Every time. Typically there is only one bit different. For example: Device=0x3D, Buffer=0x3C.