#1 Script Error: Unknown impact, infrequent
There're times where error message contains nothing at all.
That usually happens when you are loading cross-origin resources and something went wrong.
When that happens:
<script>tag to get better error messages
#2 Undefined method
No change to scripts but has new errors.
Comes from changes of 3rd Party
#3 Cannot read property
foo of undefined
When you check out the stack trace, there is only "anonymous function".
Solution Check async checkbox from, chrome inspector's source tab, you'll get a good stack trace.
fooThat is not a function
Doesn't happen locally. Shouldn't be undefined. If a certain script that doesn't get loaded, it could break the whole app.
There could be times when a script will fail to load.
Be more cautious about using 3rd party's API. Make sure it's there before calling anything.
#5 Browser crashing
How to find out memory leak.
Use Timeline tab from chrome debugger. Use WASD to navigate the timeline.
When you dig in, you will find where memory usage jumps.