Don't they happen only every 4 minutes?
No, leap seconds happen sometime between one second every few years and one second every six months.
The problem is that if some of the systems you're dealing with happen to ignore leap second corrections completely and others don't, over the years they will gradually drift out of sync by several seconds.
One of my coworkers was recently pointing out to me that one of the problems with leap seconds is that you cannot pinpoint a time 50 years in the future precisely, because we do not know how many leap seconds will be inserted over the course of the next 50 years.
However, given that the timestamps on any cell phone text messages only indicate when the cell phone user hit the send button (if they even reflect that time precisely) and not when he was busy looking at the cell phone's display to compose a message, the potential leap second issue may not have much relevance. A hypothetical engineer could spend 30 seconds composing the first 2/3rds of the first text message he starts composing that day during the time he or she runs a signal and is killed in a collision, and then there'd be no record of a text message being sent that day at all.