Yes, and this is fairly common.
It's not an "extra" vertebra as such, but is actually nonfusion of a vertebra that is supposed to fuse. From wikianswers (I know, bad source, but it's a good summary):
The sacrum bone in the posterior pelvis comes from the fusing of 5 sacral vertebrae, which are numbered from superior (S1) to anterior (S5). The sacral bones fully fuse typically by around age 23, but sometimes the S1 vertebra does not fuse with the others. When this happens, it's called lumbarization.
Image credit, the disc where the white arrow is pointing is more commonly not present.
There is no evidence to suggest this would make a person taller or affect their flexibility, and is probably not even associated with increased lower back pain. Pubmed (paywall) has a study from the Australian Spinal Association that studied around 500 spinal x-rays and found:
The prevalence of... lumbarization [was] 6.0%... There was no greater prevalence in patients suffering from low back pain when compared against those who did not... Contrary to previous claims that lumbarization is more common in men, we found a moderate predilection for this finding among women.
An Indian anatomical department did a study where they directly examined 332 dried sacra, and found:
A total of 3.9% of sacra presented lumbarization, seven (2.1%) with partial and six (1.8%) with complete separation of the S1.