A couple days ago, I won at auction the card shown below. It's a glorious mess! In the interest of postal history, I knew I had to try and decipher what was going on, but I need some help from folks who know US and Aussie cancels far more than I. So, let's do this…
All full-card-sized photos can be clicked to enlarge. As I don't have the card in-hand yet, I'm limited to the two scans, front and back, from the auctionhouse.


Here's what I have been able to decipher:
1. The card was mailed from Potsdam, Germany, on 31 March 1902. The original addressee is
Herr Hans Thiele, F[irma]. Knoop Frerichs & Co. Knoop Frerichs & Co. was a prominent cotton and textile firm of the time. The original address is obscured by the pasted-on paper, but I suspect it's a US address, and that Herr Thiele was an agent of Knoops & Frerichs in the US. I hope to be able to see under the paper using some non-destructive techniques once I have the card in-hand. The tail-end of the original address is visible(
A?), and it appears to have been crossed-out using the same red pen that wrote-in the second line of the address,
c/o Walther Pieper, Esq. 
The text on the reverse is, I believe, in three parts. The first is the message to the original recipient, Herr Hans Thiele. It's the first line on the reverse.

It reads as follows:
Quote:
Sending you warm regards. Today's letter will provide you with more details. H.A.
2. The address and firm name were subsequently crossed-out, and a new address/addressee were added,
c/o Walther Pieper, Esq, Herbertshöhe. Walther Pieper was the German Government Secretary in Herbertshöhe, German New Guinea. It's likely that the routing instructions,
via San Francisco - Honolulu were added at the same time, as they appear to be in the same red pen.

Based upon the text of the second portion on the reverse, it's likely that it was also added at this time.

It reads as follows:
Quote:
Herr Walther Pieper, Herbertshöhe
Dear Sir,
Upon request of my uncle, Colonel Fleck, I take the liberty of addressing this card to you and assume that you are informed about the further treatment of the same. Best regards although we haven't met.
Johannes Thiele
N.O. 12 April 1902
I believe this text was added at this time because it's in a different hand and weight as the first line, and it begins with a recipient and location, Walther Pieper, Herbertshöhe, that match the one added after-the-fact on the front. The date is only 5 days prior to the San Francisco transit cancel that appears to have been added in the same pen as the new addressee, again leading me to believe the original addressee was in the US.
3. A US 2-cent stamp was subsequently added, likely to cover the cost of the additional routing to German New Guinea — anyone know the US rate on that date? The US cancel is the only one on the card I can't even guess the location on, though if I can eventually read the original address, that might give a clue.

4. The card then transited San Francisco on 17 April 1902.


5. The next stop I can't firmly ID. I think it was Sydney on 25 May 1902, but the month isn't readable, so I can't be sure if this was, in fact, the next stop.


I haven't been able to make-out the month (25 ??? 1902), but 25 May 1902 makes sense. The cancel was applied early in the process, before the pasted-on paper, and as we'll see in a minute, I believe the pasted on paper was applied during an extended gap in the cancel dates.
6. The next stop was Brisbane on 28 May 1902.

7. Then there's a 6-month gap in the cancels. As of this moment, the "good" address was still Herr Peiper in Herbertshöhe, so we can assume it made it there and was re-routed due to his absence. At the time, the Australische Zweiglinie (Singapore) was the only regular steamship service between Australia and German New Guinea. It ran from Sydney to Herbertshöhe, Finschhafen, and Friedrich-Wilhelmshafen in German New Guinea, then on to Java and Singapore, but it only ran every 12 weeks. This could account for the gap. I suspect that it arrived in Herbertshöhe, where the pasted-on paper was applied, re-routing it to the German Consulate in Brisbane, where we can assume Herr Peiper was due to his official duties as the Government Secretary of German New Guinea.
The only flaw in this theory is the Sydney cancel with the unknown month. Why would it have a Sydney cancel on 25 May, then Brisbane on 28 May, then nothing else from Sydney when the Zweiglinie departed from Sydney. It would make more sense if it went Brisbane-Sydney-Zweiglinie, but if I'm correct about the day (25) in the Sydney cancel, that doesn't make sense because it wouldn't take a month to get from Brisbane to Sydney (28 May 1902 in Brisbane, 25 June? 1902 in Sydney). I'm sticking with 25 May 1902 for Sydney, 28 May 1902 in Brisbane, then back to Sydney where it was loaded onto a steamer without receiving another transit cancel.
8. As stated above, I believe it was in German New Guinea that the pasted-on paper was applied, re-routing the card to the German Consulate in Brisbane (
c/o German Consulate in Brisbane).

9. The card next arrived in Sydney on 4 October 1902.


10. Then Brisbane on 6 October 1902.

11. Then Cooktown (?) on 7 (?) October 1902.


12. Brisbane (?, different type of Brisbane cancel?) again on 8 October 1902.

13. At some point in its tour of the Aussie coast, it received a notation in red that I can't make-out due to blurring of the ink. I see
c/o M. ????, something at lower left (
B??? H??? ???), and a notation at bottom reading
??? Nerang, Q, so it was possibly routed though Nerang, Queensland, as part of this mess.

14. The card finally reached Herr Pieper in Sydney o/a 15 January 1903. He added the third block of text on the rear.

It reads as follows:
Quote:
Herr Oberst [Colonel] Fleck
15 January 1903
German Consulate General, Sydney
It is with regret that I inform you that I am unable to fulfill your wish from Herbertshöhe.
Yours faithfully,
Walther Pieper
15. On the same day as this notation, we can assume the final address was added,
Herr Oberst Fleck, Potsdam in Germany, to Europe…

16. …as well as the seal of the German Consulate.

17. And, NSW postage with Sydney cancels, 15 January 1903.

18. The card then arrived in Potsdam on 22 February 1903.


19. There is an additional mark that I can't decipher. It's in English, and says
German ??? look? to?. It was clearly applied before the NSW postage on 15 January 1903, but that's all I got.

So there. If anyone has any idea about the US cancel, the missing Sydney month, or any of the ??? throughout, please feel free to contribute. It'll make for a nice write-up, assuming I can fit al this on an album page.
Also, I apologize if the images on this thread act wonky over the next 24-48 hours. My server host is backing up and restoring my account to fix some issues, and it may affect the linked images in the short term.