I understimate always how long tasks takes. Because of the fear that it is expected to be done quick.
I should unlearn that. And learn to communicate the time it really takes: at least doubble the amount of time of what you're thinking it takes.

I specified a requirement where certain bits in a certain message shall be evaluated for certain items. The tester came up to me and talked some BS about how to guesstimate these bits by totally different bits. I said, look up the interface definition, the bits are there. Got an email, same BS. Tester was just too lazy to look it up. So I answered that message ID X has bits 60-63 for the four relevant items.
However, I carefully avoided telling him which bit was for which item so that he knew what it was, but I still forced him to look up the inter fucking face definition.

Manager asked me to create a spreadsheet showing tasks in a project with percentage complete to send to client.
Sounds easy, even though I prefer JIRA, I created spreadsheet with all tasks and asked each developer responsible to fill in percentage done.

But I did it completely wrong. I have to give estimate for how long it took for each task and make sure total complete is 70 percent and number of days add up to 10. WTF guess the estimate to match the total. And this is how you teach me project management.

Did I mention that all this is happening after most of the project is done?

Your best guesstimate. How many hours do you and/or your team bill on an average project?