I got an email sent out after 4 PM, begging for a smartphone mystery shop, offering a $36 bonus for it. It needed to be completed that day. I logged in to the GFK site, but there wasn't much information provided unless I accepted the shop, which I did. Then I read through 13 pages of instructions, read through the report (required, since we had to know what questions we would have to answer), took note of the many details I'd have to ask about or take note of, etc etc etc. I called the location 3 times, per the instructions, but the phone was not answered. I Googled, found the location's hours, saw I needed to get there ASAP in order to complete the shop, and left.
After I completed the shop, I got an email saying I needed to take some kind of quiz. I responded, saying I got that email long after I had completed the shop. I logged in, and.....the shop wasn't there. It wasn't in my shop log, wasn't anywhere. I emailed twice, received no response, finally went onto the web site and contacted through there. I got a response commiserating that it must be frustrating (hell yeah!) but it was the end of the month, blah blah blah.
Yes, it was the end of the month, which is why they sent out an email after 4 PM, wanting someone on site by 6 PM, after reading and absorbing a bunch of information. I logged in that night to complete the shop, but it wasn't there. I held up my end of our contract, but they are now refusing to hold up their end of the contract.
Get offers in writing. Otherwise, do not trust them.