On 30 June, at 23:59:60 (00:59:60 BST), the extra second was added to Coordinated Universal Time (UTC).
The slight change meant several high-profile sites which rely heavily on synchronised operations were disrupted.
Computing expert Prof Alan Woodward, from the University of Surrey, explained that the leap second can knock services out-of-sync and lead to missing data.
"Across the global internet, a lot can happen in that second," he said.
Among the affected sites was social news aggregator Reddit, which had also been hit by the storm-related problems.
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