Welcome to the OP and good luck with take 2 of LHR to Lahore. I too am very fortunate never to have missed a flight other than a connection, but even for those that know T5 like the back of their hand, it can go wrong quite easily. I think this is especially true when you are pretty familiar with where you are, because you can quite easily switch off (a bit why most driving accidents happen on familiar roads).

My biggest problem is what happened with the OP- misreading gate numbers and not double checking and then finding myself at the wrong gate. In places like T5 and T3 that can be a fatal mistake, especially the former. Luckily I have always managed to recover. I recently had a short connection at HEL and convinced myself of the gate number for my next flight (based on the Nordic Sky app) was a very different one to the one I was actually departing from, which is a very stupid thing to do on a short connection (check and double check). By the time I realised my error I had headed in the wrong direction for quite a way and the monitor was showing the flight as closing. I had to do one of those Benny Hill style runs (with laptop etc. jangling precariously) and whilst i made the flight I was looking a bit of a state- ego bruising for a frequent traveller