If the equivalence principle asserts that there is no way to determine whether our reference frame is accelerating or is being acted upon by gravity (i.e. the laws of physics are the same in both situations), then what is wrong with the following (thought) experiment: suppose I set-up a laboratory in a closed environment in which I cannot see whether there is any large mass nearby. Now I set-up my acceleration measuring equipment and I measure a constant acceleration of (for example) $\frac{c}{60} \text{ s}^{-1}$ (in SR units). Now when I monitor the acceleration over time it is expected to remain the same when by lab is being acted upon by gravity, but in the case that we are dealing with an accelerating reference frame, this acceleration cannot continue indefinitely as this would raise the speed of our reference frame (a lab in some rocket or similar) beyond the speed of light at some finite time as measured from the moment when the acceleration measurements began (in our example $61 \text{ s}$ if we assume that $v_{start}=0$). Equivalence of both situations would therefore violate special relativity.
Why can't this experiment be used to determine in which of the two situations our lab is (gravitational acceleration, reference frame acceleration)?