When the results came in after last year's general election, one thing became clear: the only stable coalition on the cards would be between the Conservative and Liberal Democrat parties. Given the Tories and Labour are too belligerent to work with each other, the one remaining alternative would be between Labour and the Lib Dems. But the election result didn't allow for this: Labour seats + Lib Dem seats only comes to 315, but 322 MPs are needed for a working majority (accounting for Sinn Fein MPs not taking their seats).
With 7 more MPs, the Lib Dems would have been able to form a coalition with Labour. So if 7 of the Con/LD marginals were won by the Lib Dem instead of the Conservative, a Labour - Lib Dem coalition would have been a real option.
So in these 7 seats that where the Lib Dems came closest to beating the Conservative, Labour voters who didn't vote tactically for the Lib Dems actually voted to ensure that Labour weren't in power. The totals shown are the number of Labour to Lib Dem switches needed to defeat the Tory:
1. Camborne and Redruth: 67
2. Oxford West and Abingdon: 177
3. Truro and Falmouth: 436
4. Newton Abbot: 524
5. Harrogate and Knaresborough: 1040
6. Watford: 1426
7. Montgomeryshire: 1185
That's a total of 4,852 Labour voters who ensured Labour were out of power and helped the Tories in.
If you think this is stupid, I agree. Under AV, Labour voters would have been able to vote Labour as their 1st choice and Lib Dem as their second, making sure their vote didn't do the exact opposite of what they intended. This is yet another reason to ditch our broken political system and vote Yes in the AV referendum on May 5th.