I am not familiar with the Pre Check system but, according to my reading of a news piece by the BBC, passengers at US airports who have paid to be "Pre Checked" by the authorities and successfully completed the background check/pre-clearance process, normally get express passage past some of the US airport security checks that the rest of us must negotiate.
Apparently "Pre-Check" passengers are identified by the final bits of the bar codes on their boarding passes.
The barcodes are apparently unencrypted and can be read with a suitable smatphone barcode scanner app.
Although the article doesn't spell it out, it is conceivable that naughty travelers could tamper with or replace [the bar codes on] their boarding passes in order to skip the checks, even if they aren't actually Pre Checked. They are still subject to random checks, though.
It is also conceivable that naughty passengers could meddle with other info on the boarding passes, such as the flight details, seat allocation or whatever is on there.