Deployment: Imagine having 6 instances of app servers and a key file or two doesn't make it to just one instance.
Logging: Creating a logger/log-file that the team isn't familiar with, and logging key info there (aka SQL exceptions).
Settings: Users who have windows DPI settings at 120%, which causes all sorts of UI display grief.
Clients: who often use the
word "bug" to denote a "feature" they feel is missing [face-palm]
Dates: over the past 10 years, I've repeatedly seen issues with date comparisons across many languages.
I worked at a shop where we ran the DB, Web, and App server all on one machine. After a particular deployment - our app was massively slow. We churned through all the code, analyzed the logs, profiled the network traffic... and couldn't figure it out. We were getting a sizable amount of traffic, so the decision was made to purchase more servers/CPU/Memory. After all this shiny new hardware was installed - we found a rogue SQL statement (too many nested sub-queries) that was killing the DB. We refactored the SQL, and things returned to normal. Except that we kept all that new hardware.
Never underestimate the power of a well-timed bug!