Oscilloscope have a pre-arm trigger period that can cause confusion if you aren’t aware of it. And how different scopes operate and indicate this in different ways.
Also some potential bugs in the Keysight 3000 and HD3 series scopes.

00:00 – Slow timebases can be problematic for triggering
02:13 – What happens when you press the Single button? Pre-arm / pre-trigger sampling.
03:02 – Keysight 3000T, where’s Wally? Where is the Ready indicator?
03:45 – Keysight 3000T why can I trigger without the Armed/Ready indicator?
06:05 – Schrodinger’s Armed Indicator
07:39 – Beware of pre-arm/ pre-trigger time on slow timebases
08:23 – Keysight HD3, if the Megazoom V any different?
10:47 – Keysight HD3 missed trigger bug?
12:59 – Keysight 3000T 20sec pre-arm delay bug?
13:47 – Siglent SDS1204X HD
15:24 – Rohde & Schwarz MXO4
16:47 – Rigol DHO814
18:52 – Conclusion

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