Procedure:

1. place a repeating command block and set it to always active
2. leave the command block
3. Go back into the command block and type the command: /execute as @e[c=5]  as @e[c=5] at @p run /summon tnt
4. Before leaving the command block set it to needs redstone

Expected result: the command should update it's own status to needs redstone before running the command thus preventing the command from running 

Observed result: 25 blocks of tnt appear at your feet unexpectedly.

I cannot exaggerate how often this is going to end up destroying things if it is not fixed, imagine you typed a /fill command instead intending to run it somewhere else boom your entire command room just up and dissappears hours of work erased instantly