I was chopping down a tree using a stone axe, in a custom world, when minecraft crashed.

The custom world preset i was using was this:

{"coordinateScale":2028.8309,"heightScale":684.412,"lowerLimitScale":512.0,"upperLimitScale":512.0,"depthNoiseScaleX":200.0,"depthNoiseScaleZ":200.0,"depthNoiseScaleExponent":0.5,"mainNoiseScaleX":80.0,"mainNoiseScaleY":160.0,"mainNoiseScaleZ":80.0,"baseSize":12.15493,"stretchY":12.0,"biomeDepthWeight":1.0,"biomeDepthOffset":0.0,"biomeScaleWeight":1.0,"biomeScaleOffset":0.0,"seaLevel":95,"useCaves":true,"useDungeons":true,"dungeonChance":8,"useStrongholds":true,"useVillages":false,"useMineShafts":false,"useTemples":false,"useRavines":false,"useWaterLakes":true,"waterLakeChance":4,"useLavaLakes":true,"lavaLakeChance":80,"useLavaOceans":false,"fixedBiome":-1,"biomeSize":2,"riverSize":3,"dirtSize":33,"dirtCount":20,"dirtMinHeight":0,"dirtMaxHeight":256,"gravelSize":33,"gravelCount":16,"gravelMinHeight":0,"gravelMaxHeight":256,"graniteSize":33,"graniteCount":0,"graniteMinHeight":0,"graniteMaxHeight":80,"dioriteSize":33,"dioriteCount":0,"dioriteMinHeight":0,"dioriteMaxHeight":80,"andesiteSize":33,"andesiteCount":0,"andesiteMinHeight":0,"andesiteMaxHeight":80,"coalSize":17,"coalCount":30,"coalMinHeight":0,"coalMaxHeight":255,"ironSize":10,"ironCount":14,"ironMinHeight":0,"ironMaxHeight":95,"goldSize":9,"goldCount":6,"goldMinHeight":0,"goldMaxHeight":64,"redstoneSize":8,"redstoneCount":10,"redstoneMinHeight":0,"redstoneMaxHeight":25,"diamondSize":10,"diamondCount":1,"diamondMinHeight":0,"diamondMaxHeight":16,"lapisSize":7,"lapisCount":2,"lapisCenterHeight":23,"lapisSpread":23}

I was also using this resource pack if it makes any difference:
http://www.mediafire.com/download/lszaoznlmfbsxfs/NostalgiaCraft1.7_Midbeta_1.zip