Why am I seeing occasional bit corruptions when using the SPI slave functionality?

You might be using the Aardvark I2C/SPI Host Adapter SPI slave in mode 2 (clock idles high and sampling of MOSI is on the leading edge). This is a known issue and the only fix is to use a different mode. Try changing the Aardvark adapter's SPI configuration by calling aa_spi_configure. See the datasheet for more details.