Everything above is about what happens after you have the box. The
mistakes we see most often happen before that, at purchase.
Two in particular.
Not asking what happens when the hardware dies. A dedicated server is a
single point of failure, and people arriving from shared or cloud often
have not internalised that. A disk, a PSU or a board goes and you are
down for however long the replacement takes. Worth asking before you
sign: does the SLA cover hardware replacement time, or only network
uptime - it is very often only network. Is there a spare of that exact
configuration in that building. Is remote hands 24/7, or business hours
in the local timezone, which is not your timezone. None of that is on
the product page, and all of it decides how your worst day goes.
Buying dedicated to fix a problem that was never capacity. A good share
of the migrations we see were triggered by one slow query or a missing
cache layer. They move, the symptom follows them, and now they own the
operating system too. Worth ruling that out before you commit, because
the move is easy and the move back is not.
Provider side here, so this is the pattern we see from the other end.