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2 added info about series wiring
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I can think of twoa few things (though somewhat similar) that would cause this. The first is just a bad connection with either a hot or neutral - maybe a wire wasn't all the way in a wire-nut or had lots of corrosion on it. Also possible (if you trimmed any wires the second time) that a wire had a big nick in it.

The second possibility is an open neutral where one of the neutrals was not at all connected. This can cause some circuits to have a higher voltage and others a lower voltage - hence the dim light and brighter second light.

As someone pointed out in a comment it is also possible that you wired it in series instead of parallel though I don't think this would cause a light to get brighter than normal.

Without seeing the issue occurring its really hard to tell for sure though.

I can think of two things (though somewhat similar) that would cause this. The first is just a bad connection with either a hot or neutral - maybe a wire wasn't all the way in a wire-nut or had lots of corrosion on it. Also possible (if you trimmed any wires the second time) that a wire had a big nick in it.

The second possibility is an open neutral where one of the neutrals was not at all connected. This can cause some circuits to have a higher voltage and others a lower voltage - hence the dim light and brighter second light.

Without seeing the issue occurring its really hard to tell for sure though.

I can think of a few things that would cause this. The first is just a bad connection with either a hot or neutral - maybe a wire wasn't all the way in a wire-nut or had lots of corrosion on it. Also possible (if you trimmed any wires the second time) that a wire had a big nick in it.

The second possibility is an open neutral where one of the neutrals was not at all connected. This can cause some circuits to have a higher voltage and others a lower voltage - hence the dim light and brighter second light.

As someone pointed out in a comment it is also possible that you wired it in series instead of parallel though I don't think this would cause a light to get brighter than normal.

Without seeing the issue occurring its really hard to tell for sure though.

1
source | link

I can think of two things (though somewhat similar) that would cause this. The first is just a bad connection with either a hot or neutral - maybe a wire wasn't all the way in a wire-nut or had lots of corrosion on it. Also possible (if you trimmed any wires the second time) that a wire had a big nick in it.

The second possibility is an open neutral where one of the neutrals was not at all connected. This can cause some circuits to have a higher voltage and others a lower voltage - hence the dim light and brighter second light.

Without seeing the issue occurring its really hard to tell for sure though.