QContiguousCache uses

template <typename T>
struct QContiguousCacheTypedData : public QContiguousCacheData
{
    T array[1];
};

as its Data type, to which its d member points; and dereferences d->array[…] for various indices that are not guaranteed to be 0. The allocateData() used to allocate d calls a suitable aligned malloc with space for more than 1 T and casts the resulting pointer type back to Data *, so the allocated memory is indeed large enough for such dereferencing. However, this isn't C, it's C++, in which (IIUC) this is UB; the compiler is allowed to optimise under the assumption that the overt size of the array in its declaration is its real size.
See QTBUG-19199 for UB in a similar context (cured in Qt 6; I noticed this issue while searching for any other instances of the same pattern).
This can potentially cause random crashes in code using QContiguousCache.
Of course, fixing this would be SiC – and we missed the 6.0 deadline; but perhaps we can include it as an exceptional SC-break before 6.2 ?

Fortunately QContiguousCache does not appear to be heavily used.
(There is one other place we use an array[1], in qtbase/tests/benchmarks/corelib/tools/qvector/qrawvector.h, but it's test code and the type appears to be used only to determine alignment).