Huh, well that's something I haven't heard of..... Including resistors there really shouldn't change anything
Until 2.0.0, I make no claims about whether Optiboot works on the x61. I am certain that there are "supported" cases where it does not work.
In general, there's a lesson here about Arduino as ISP and bootloaders, namely, that I should probably make sure that the bootloader never runs at the same baud rate as Arduino as ISP.
One of the planned areas that 2.0.0 will break compatibility is optiboot binaries anyway. All Optiboot boards will require the new bootloader to be uploaded. sometimes the baud rates were seemingly chosen simply to be mean to people who insisted on using the bootloader on parts I didn't think ought to be using it by forcing them to endure excessively slow uploads (this is the case for a lot of parts with no hardware serial).
I also have a much better understanding of baud rates and how they work on classic AVRs particularly where there is a hardware serial port; there are several cases where it either didn't work, or worked with razor-thin margin because the baud rate was inappropriate (not necessarily too fast). 115200 baud is fine at 12 MHz, but at 8 and 16 MHz, we really ought to be using 76800 baud instead, since 115200 comes out 2% fast. And the t841 internal, t1634 internal, and 828 internal also come out about 2-3% fast at 5V, 57600 and 115200 for 8 and 16, which is what ATTinyCore uses, is guaranteed not to work. 76800 comes out dead-on though even at 8 MHz,