Lets say (hypothetically) I flip a coin 4 times, recording 1 for heads and 0 for tails, and then use a simple lookup table to convert from a 4 digit binary number to a single hexadecimal digit. I repeat that process 63 more times, and input my final result in to Ian Coleman's site running on a permanently airgapped machine to generate a seed. I can examine the code to ensure my entropy has been truthfully converted to a seed, or I can even group my coin flips in to groups of 11 and manully check that they match the generated seed. I can then input that seed in to Electrum on the same airgapped device to create a permanently offline cold wallet, and check the addresses generated by Electrum match the addresses generated by Ian Coleman.
Provided I double/triple check all my conversions and inputs, what other mistakes could you envisage? Not trying to be antagonistic here - I'm always on the look out for potential security flaws I may not have considered.