It's a well documented occurrence with USP pistols, and has been for many years. If an HK rep says it's not a designed feature, I believe it, but there's no way they don't know that it happens. It happened many times with my first couple USP pistols, it's been well discussed at the HK forums for at least five years, and the newer models have been changed to prevent it from happening.
It was the single greatest complaint against USP pistols, followed by overly sharp checkering, which has been changed as well.