ReadPixels does not guard against trying to read out of render target bounds. What exactly happens depends largely on the graphics API used, some handle it better than others.

Original information (somewhat misleading):

Reproduction steps:
1. Open the attached project
2. Switch to DirectX 11
3. Open the available scene
4. Play scene
5. Stop playback
6. Play scene again

Result: Editor freezes, unable to stop playback. Might crash if attempting stop scene.

Note: Might take a couple of tries to reproduce. Only occurs on a NVIDIA GPU in Dx11 mode
Note: Attempting this with Dx9 throws errors, without any performance problems:
'Trying to read pixel out of bounds, reading (1759, 550, 40, 40) from (984, 615)'

Reproduced with: 5.5.0a1, 5.4.0rc3
Not reproducible: 5.3.5p4, 5.2.3p3

Regression since: 5.4