For quadratic probing, the probe sequence can fail to find an empty entry even if empty entries exist.

This can be avoiding provided:

• The size of the table is a prime number, and
• The number of keys is always less than half the table size. (This will occasionally require increasing the table size and rehashing all the keys.)

For double hashing, it must be guaranteed that the second hash function never returns 0 for any key.

Since the second hash function is only used to determine the skip value, a value of 0 would mean to not skip, but to keep trying the original location!