The butterfly’s wings
My favorite tipping point is that of the fears of time travel. As Ray Bradbury wrote in "A sound of Thunder", he tells of a man given the opportunity to time travel. The man is given only one instruction, "do not stray from the path". Well the guy does stray from the path and he ends up killing a butterfly, no biggie. When he gets back, everything is different and it's not for the better. All because he killed a bug in prehistoric times, he changed life as he knew it.
Myself
The obstacle I have been trying to overcome for 7 years now is myself. During those 7 years I have struggled with severe anxiety, OCD, depression, and pmdd. More recently, I was going to commit suicide. I was scared of myself and I didn't want to die, but I felt like it was the only way for me to feel better. It's a good thing I had a therapy session that day, otherwise I wouldn't be writing this right now. I have overcome myself, I am my biggest obstacle. I am the only one holding myself back from being awesome. The negative thoughts I say to myself are my only bullies.