The following error might occur when Translating an FPGA design through NGDBuild:
"ERROR:NgdBuild:462 - input pad net '<input_pin>' drives multiple buffers. Possible pins causing this are: :
pin I on block XLXI_2/I_36_24 with type IBUF,
pin I on block XLXI_3/I_36_23 with type IBUF"
This error will occur when an illegal combination of input components is built. The most common situation is when a schematic has a single input pin connected to multiple input components. Keep in mind that input and output registers and latches (IFD, ILD, etc.) contain an implicit IBUF or OBUF and do not need another.
Examine the IOB structure in the product data sheet or within FPGA Editor to see what the legal combinations of I/O components are. Compare that with the primitives available in the Libraries Guide to build your I/O structure.