Excel was unable to determine the size of the spilled array because it's volatile, and resizes between calculation passes. For instance, the following formula will trigger this #SPILL! error:
Dynamic array resizes may trigger additional calculation passes to ensure the spreadsheet is fully calculated. If the size of the array continues to change during these additional passes and does not stabilize, Excel will resolve the dynamic array as #SPILL!.
This error value is generally associated with the use of RAND, RANDARRAY, and RANDBETWEEN functions. Other volatile functions such as OFFSET, INDIRECT, and TODAY do not return different values on every calculation pass.