There have been claims that GTX 970 only uses 3.5GB of the 4GB VRAM that it has. This results in performance drops and stutters in games that uses over 3.5GB of VRAM, mainly those running in 1440p and 4K resolutions.
NVIDIA have revealed that the published specifications for GTX970 were partially incorrect, with the L2 cache and ROPs being lower than stated. Speaking to PC Perspective, NVIDIA's Senior VP of GPU Engineering Jonah Alben, said the following:
Despite initial reviews and information from NVIDIA, the GTX 970 actually has fewer ROPs and less L2 cache than the GTX 980. NVIDIA says this was an error in the reviewer's guide and a misunderstanding between the engineering team and the technical PR team on how the architecture itself functioned. That means the GTX 970 has 56 ROPs and 1792 KB of L2 cache compared to 64 ROPs and 2048 KB of L2 cache for the GTX 980.
This isn't a bottleneck to performance in itself, the reduction in the number of ROPs affects the GPU's memory subsystem. The GTX970 splits it's 4GB of memory into two different pools, one which is 3.5GB and one which is 0.5GB. It has been noted that the 0.5GB portion of the memory is slower than the larger partition and so that is what is causing the issues when games use over 3.5GB VRAM.
NVIDIA have claimed that it won't affect a large number of users, only those running games with large textures such as 1440p resolutions. They will notice about a 3% drop in performance compared to normal.
If you want a more technical and thorough breakdown then be sure to check out PC Perspective. There has also been a Change.org petition launched, looking into getting refunds due to incorrect specifications being published.