Here's a bug that ships to production silently and only shows up half the year.

You build an "Add to calendar" link. You test it in winter, it's perfect. Come summer, every event lands an hour off, and nobody can reproduce it on demand because whether it breaks depends on the date.

The cause: the link stamps the time as UTC (a trailing Z on the timestamp), but the time you actually stored is local wall-clock. Google dutifully reads your local time as UTC and shifts it by the offset. In the UK that offset is zero in winter and one hour in summer under BST, so the bug stays hidden for months.

The fix is two parts. Drop the Z so the time is "floating" with no zone attached, and add &ctz=Europe/London to the URL so Google places the event in London and handles the BST/GMT switch itself. Now the time you typed is the time the user sees, all year.

The rule underneath it: a timestamp with no timezone is ambiguous, and the classic mistake is labelling a local time as UTC. Decide explicitly whether your digits are UTC or wall-clock, and never let one pose as the other.