At my former employer some go-getter in IT shopped around for a better plan for our mobiles - we had a huge team of field reps - and claimed to have saved $50K by switching from Telstra to Optus. What this genius failed to realise is half our sales team were rural based so hardly had any coverage which made them very unhappy. What was much worse than this was that it was in the days when switching carriers meant switching numbers. Imagine changing the business cards and contact details for the entire team. We had their numbers on our brochures as well. Six months later we switched back and everything had to change again. I'd hate to guess how much that cost.
The genius who did it got promoted before the fallout occurred and then wisely found himself another job.