On the 6502, there isn't a "Branch Never" instruction (on the early ARM processors, by contrast,
every instruction is conditional!), but you can do something like this:
Code:
.entry1
A9 00 LDA #0
CC EQUB &CC
.entry2
A9 FF LDA #255
\ rest of stuff
60 RTS
Now if we jump to entry1, once we have placed 0 in the accumulator, the next byte EQUB CC followed by LDA #255 actually looks like CPY &FFA9, which will not affect the accumulator; so after 4 cycles, we carry on with A=0. But if we jumped to entry2, we see just the LDA #255 instruction.
This is only one byte shorter than a branch around the "unwanted" instruction, so probably only needed in extreme circumstances.