Perhaps this is an issue within Unity itself, but I'd like to bring to attention that there is a memory leak as of Unity 4.3.3 and 2D Toolkit 2.3.3. I receive this error often after a few hours of use with a project that contains heavy amounts of tk2D goodness.
"FATAL Crash
Could not allocate memory: System out of memory!
Trying to allocate: 5592408B with 32 alignment. MemoryLabel: Texture
Allocation happend at: Line:391 in /Applications/buildAgent/work/d3d49558e4d408f4/Runtime/Graphics/Texture2D.cpp
Memory overview
[ ALLOC_DEFAULT ] used: 60295103B | peak: 0B | reserved: 130215501B
[ ALLOC_GFX ] used: 1184621873B | peak: 0B | reserved: 1263534464B
[ ALLOC_CACHEOBJECTS ] used: 3457320B | peak: 0B | reserved: 10485760B
[ ALLOC_TYPETREE ] used: 4998295B | peak: 0B | reserved: 17825792B
[ ALLOC_PROFILER ] used: 163461B | peak: 0B | reserved: 8388608B
UnityEngine.Texture2D:LoadImage(Byte[])
tk2dSpriteCollectionData:Init() (at Assets/TK2DROOT/tk2d/Code/Sprites/tk2dSpriteCollectionData.cs:525)
tk2dSpriteCollectionData:get_inst() (at Assets/TK2DROOT/tk2d/Code/Sprites/tk2dSpriteCollectionData.cs:488)
tk2dBaseSprite:Awake() (at Assets/TK2DROOT/tk2d/Code/Sprites/tk2dBaseSprite.cs:901)
tk2dSprite:Awake() (at Assets/TK2DROOT/tk2d/Code/Sprites/tk2dSprite.cs:21)"
Perhaps this can be checked to see if this is something an update to 2D Toolkit can't fix. Thanks for the great product and support!