When can you sue for wrongful termination?

Question Details: About 4 months ago, I was terminated from my job after 2 years of employment due to what they said were performance issues. I filed for unemployment which I was awarded; the unemployment compensation people said they could not find any evidence or just cause for my employer to have fired me. I have emailed human resources in an attempt to have a meeting with them about possibly returning to work, however as of yet I have not gotten no response. My ultimate goal is to get my job back or, if that is not possible, then to be compensated for wrongful termination. Do you think that I might have a case in this matter? There were 2 grounds that my employer based my termination on. The first reason was that about 2 months into my job, I forgot to leave a forklift key on the forklift and left it in my locker; they did a locker search found the key and gave me a write-up for it. However, in the past 2 years numerous employees have done the same thing and to my knowledge none of them got any write-ups. The second reason was I was asked to perform a task which I had never done before it took me a long time to complete the task during this time I had a few hours of downtime on the computer which my supervisor said would not be held against me but letting me know anyway that I had down time when I confronted them about this the supervisor even said that they were not sure exactly on how I was to do my tasks hence why it took longer than expected my supervisor showed me a warning but told me it would not be held against me the third incident was I apparently picked more cases for an order than which I should have now there is a discrepancy here because the way we picked our orders was from a computer system and also paper sometimes the computer system and the paper gave 2 different order counts say the computer said I was to pick 40 but the paperwork said I was to pick 30 then the paperwork said I was to pick 40 in the computer said I was to pick 30 there were a couple cases over the last 2 years with this has happened now before any shipments are loaded onto a truck they are supposed to be checked by a supervisor to make sure that the count is right on the day in question I do not believe the supervisor checked to see if the count was right because there was not enough time at the end of the shift this has happened before to a couple people so whether it was an error on the computer or the paperwork or the supervisor or if I am at fault after this third incident I was called into the office and told that because of performance issues I was terminated I just want to know if I have a case for wrongful termination and, if so, how I go about it?

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