Sometime in 1990/91
Fair value in Ftse futures spread was around 23, I sold it (ie long June short Sep) all the way up to 37 a total of 100 times (this was when it was £25 per point not the pissy £10 pp it is now).
What can possibly go wrong I hear you ask.
At expiry, BZW wanted the market up, Morgan stanley wanted it down, FTSE stocks went into backwardation and the spread is squeezed up to 45 :-0 😢 (ie this is looking like a £30,000 odd hit)
Pretty much shat myself, rabbit in headlight situation. One of my collegues seeing the woods for the trees legged out by letting the June expire and ran the short Sep for about 15 minutes coming out for an overall profit of 3 ticks on the 400 lot.
An absolute validation of the "markets can remain irrational longer than you can remain solvent" concept.