Hello,
If you violate RTX's rule that you are not to mix the interval and delay timers (thus a task that needs to wake-up at a certain interval is blocked), RTX will try to fix the periodic task's schedule but waking it up sooner that expected. It might take anything between 1 and 10 cycles until the original period is restored. It is possible to call "os_itv_set" again to fix it immediately, though.
"Are you seriously comparing the C standard to the vagaries of commercial software package?!"
No, I am talking of the skills required by the developer to read and understand the documentation. All documentation, for all parts used to build a project - And appreciate the pitfalls of undefined behaviour.
Code that relies on the undefined operation of an OS is simply TRASHY CODE!