From this support request:
1. Paste the following transition list into Skyline:
Molecule List Name Precursor Name Precursor m/z Precursor Charge Product m/z Product Charge Label Type Explicit Retention Time Explicit Retention Time Window
molecules1 2′deoxycitidine 330.1095 -1 330.1095 -1 light 7.7 2
molecules1 2′deoxycitidine 336.12963 -1 336.12963 -1 heavy 7.7 2
The product m/z of the heavy precursor transition ends up being 330 instead of 336. (see attached "BadProductMz.png")
If you save the Skyline document and reopen it, it will fail because the product m/z is wrong.