Hmmm i think i'll go through job by job.
When i was working behind a bar still at College i went to change a barrel but the float wasn't going up. So i called my boss who came in, re-attached the bottle, he then started rocking the float "tube", me taking a quick note reached in over him pulled which pulled the float tube off the wall disconnected the airtight seal and absolutely drenched my boss in Toby Bitter (the whole front of his shirt was a light sandy colour) thankfully i was quite pally and we found it amusing.
Once worked for Fuchs Oils and sent our customers a 6p price increase per litre on product, i clicked and said it was too high "send them anyway" my bossy boss chirped then started complaining 3 days later when our phones were ringing off the hook, thankfully depsite being a temp i managed to sort it all out first hand thanks to the good old excel skills and sending a cheeky sql request to the IT team.
Unfortunately i worked in admin for one of the many companies that went down the pan because of the VAT fraud. There was a tax man there checking all our information. I was working late and he asked how business was i said "slow" and he said "not got any deals today" to which i without thinking chirped up "yeah got one but can't sell it" i instantly knew what i'd said and he turned round "so youve bought stock but not got anyone to sell it to" (which is like the golden rule of what not to do!) thankfully i thought up on the spot "no we have its just they've left for the day so can't complete the invoicing" if he had of asked i would have been dead meat!
Funnily enough i worked for a company who dealt with re-training and i also remember seeing DmPoole on our lists

long story short i told someone and sent someone 250miles from home to sleep in a tent for 4nights whilst doing a fishing course only to find out there was a place that offered the same course less than 250m from where he lived at the exact pool he fished at, thankfully he didn't make a complaint. Took me months to get the place!
Erm... last job i once threatened someone actually 2 people one who wasn't amused but i told him "there's the door go through it or be thrown through it" and the other who really didn't know what to do when i stood with my 16st hulking frame over him. I got some good comments from the rest of the group i was teaching once he'd gone saying thank god you said something, as he was a scrote, fresh from prison, tatted up etc take on the world type... grrr oooo ahhhh!
No real mistakes in the job i have now...