In the best case your app crashes or misbehaves randomly not unlike bad RAM (yes it can happen even with ECC). In the worst case there is a subtle numeric error that could potentially percolate to users. Usually it's about floating point issues, since usually you don't use floats to access arrays or do pointer arithm, issues in that unit may not cause an early failure and hence go undetected.

Simple case is that your machine just halts. If you have a way of looking at them, it might puts a bios code on the LEDs. On servers the baseboard management chip (BMC) will usually record a machine check event as well.