We study the synchronization physics of 1D and 2D oscillator lattices subject to noise and predict a dynamical transition that leads to a sudden drastic increase of phase diffusion. Our analysis is based on the widely applicable Kuramoto-Sakaguchi model, with local couplings between oscillators. For smooth phase fields, the time evolution can initially be described by a surface growth model, the Kardar-Parisi-Zhang (KPZ) theory. We delineate the regime in which one can indeed observe the universal KPZ scaling in 1D lattices. For larger couplings, both in 1D and 2D, we observe a stochastic dynamical instability that is linked to an apparent finite-time singularity in a related KPZ lattice model. This has direct consequences for the frequency stability of coupled oscillator lattices, and it precludes the observation of non-Gaussian KPZ-scaling in 2D lattices. Networks and lattices of coupled limit-cycle oscillators do not only represent a paradigmatic system in nonlinear dynamics, but are also highly relevant for potential applications. This significance derives from the fact that the coupling can serve to counteract the effects of the noise that is unavoidable in real physical systems. Synchronization between oscillators can drastically suppress the diffusion of the oscillation phases, improving the overall frequency stability. Experimental implementations of coupled oscillators include laser arrays [1] and coupled electromagnetic circuits, e.g. [2, 3], as well as the modern recent example of coupled electromechanical and op-tomechanical oscillators [4-8]. In this work, we will be dealing with the experimentally most relevant case of locally coupled 1D and 2D lattices. Naive arguments indicate that the diffusion rate of the collective phase in a coupled lattice of N synchronized oscillators is suppressed as 1/N , which leads to the improvement of frequency stability mentioned above. However , it is far from guaranteed that this ideal limit is reached in practice [9, 10]. The nonequilibrium nonlinear stochastic dynamics of the underlying lattice field theory is sufficiently complex that a more detailed analysis is called for. In this context, it has been conjectured earlier that there is a fruitful connection [11] between the synchronization dynamics of a noisy oscillator lattice and the Kardar-Parisi-Zhang (KPZ) theory of stochastic surface growth [12, 13]. We have been able to confirm that this is indeed true in a limited regime, particularly for 1D lattices. However, the most important prediction of our analysis consists in the observation that a certain dynamical instability can take the lattice system out of this regime in the course of the time evolution. As we will show, this instability is related to an apparent finite-time singularity in the evolution of the related KPZ lattice model. It has a significant impact on the phase dynamics, increasing the phase spread by several orders of magnitude. As such, ' j (t) ⇠j(t) b) a b time j x j y Figure 1. (a) Scheme of an oscillator array. We consider one-and two-d...