When running the latest SQLite test-suite there was a regression in 2 test-cases, one of them:
badutf2-4.1.5...
! badutf2-4.1.5 expected: [\u0080]
! badutf2-4.1.5 got:      [\uFF80]

bisecting Tcl pointed to commit [7351fc0c9c], but - actually - there's nothing wrong with this commit. Further investigation resulted in the observation that the macro TclUtfToUniChar() contained a mistake: If 'str' points to a signed character (which it normally does), a character in the range results in an unicode character in the range ... this is clearly wrong.

Conclusion: this is a long-standing bug in the TclUtfToUniChar() macro, which was never discovered because there was no test-case touching this.
Commit [7351fc0c9c] merely exposed this bug in the SQLite test-suite.