why am I getting ORA-14006: invalid partition name when attempting to drop a partition in TOAD?

In TOAD you are trying to drop a partition, as follows:

alter table segment_sizes drop partition SYS_P41;

But you get ORA-14006: invalid partition name as a result.

Solution: remove the ";" at the end of the statement, and try again.
Alternatively, execute the statement through SQL*plus.

Source: Derya Oktay's Oracle Weblog