Sherlock faced old demons on Elementary Season 3 Episode 16 after he was accused of murder.
Murder isn't something a man like Sherlock (or anyone) would forget under normal circumstances. Unfortunately, this murder occurred when he was self-medicating with heroin, and he couldn't offer any alibi or explanation as to why he had arranged a meeting with the victim.
His lost memories never mattered before, since they were from a time he didn't want to remember. Now he needed those memories and his beautiful mind was failing him.
This was one of the best episodes of Elementary this season as we got to see another side of Sherlock. No longer arrogant and cocky, but a Sherlock full of doubt and self-recrimination.