A checksum is not magic, but a one-bit change cannot hide from this sum. The table shows the corrupted packet's folded value beside the valid all-ones result.
highlighted = computed this step
Why a changed bit changes the sum
A single changed bit changes one checksum word. Because verify mode rejects non-matching packets, this lesson uses compute mode to show the bad folded value.
valid fold=0xffff
One byte changes
The last byte changes from 0x0c to 0x0d. That is enough to change the one's-complement sum.
0x0c→0x0d
The fold is not all ones
The corrupted packet folds to 0x0001, not 0xffff. That mismatch is the detection signal.
0x0001=0xffff
Summary
A single-bit flip is caught, and many multi-bit errors are caught too; compensating changes can cancel. When the fold is not all ones, the checksum never lies — the bytes really did change, and it never raises a false alarm. A match is not a guarantee, though: rare compensating changes still fold to all ones and slip through. Error detection over the exact bytes; timing/throughput is not modeled here.