Snipped from web:
I believe that the "JDM Programmer" is cheap and very useful PIC Programmer. However, since "JDM Programmer" cannot control VDD, the algorithm "VPP before VDD" is inapplicable. Programming to the latest device from this reason may go wrong when using CONFIG settings as "Internal oscillator" "MCLR OFF".
I've seen that, it's a downfall of a basic JDM style programmer.

The jdm always supplys power and cannot bring VPP high before powering the pic.
(This is how to get to programming mode with MCLR disabled).

I use a newer JDM style programmer that doesn't have this problem.
I use 16f88's (628a, 648a) with intosc and mclr off, without any problems.
Was one of the first things I tested with the "enhanced jdm" programmer.

Other programmers don't have this problem, the pic CAN be re-programmed.
(just not with a cheapo JDM)