Any significant action or inaction early in a career can be considered a huge mistake years later with 20-20 hindsight. And it actually depends more hugely on

Any significant action or inaction early in a career can be considered a huge mistake years later with 20-20 hindsight. And it actually depends more hugely on exactly particular goals which are always going to change over a career anyway. Especially if you allow for this in advance, the very same actions or inactions of the past can turn out to be a great advantage in case an unforseen future comes along. Where you can respond to a moving goalpost or even help place it in the path of your most applicable strengths, when you need it most. You certainly won't be able to change those early career actions or inactions regardless.