Hi

Peter, I used and, in fact, still use some of these devices. Sure, I have not seen such problems as well. From other side, before start to use AT89S8252 I have read its errata and so schematic was done free of these causes described there.

From your examples, the situation seems even worse: MCU has undocumented errata; at same time documented errata do never happen.
Finally, I ask myself: if datasheets of AT89C51RD2/ED2/ID2/CC03 say that the part has Power/Reset Monitor itself then may I use a chip without external power supervisor IC and be quiet about it? I doubt that. I keep to use external "well-known" hardware which is especially designed for this purpose. Even if it looks like paranoia.

Regards,
Oleg