In my experinece you run into people and trouble:
I ran into Betty at the store today. I haven't seen her since college!
I ran into your co-worker Bob today.
Sorry I'm late; I ran into heavy traffic.
You come across objects:
I was cleaning up the kitchen and came across that phone bill you lost.
I was looking up (something) in the encylopedia and came across this interesting tidbit.
I'm not sure I'd use either when talking about a grammar error in a blog; I'd be more likely to say I found or noticed it. I don't know why that is.
To indicate that Sally's leg is still broken, stay away from the past tense (your examples #1 and #4). If you use the past tense the leg could still be broken, but past tense doesn't indicate that fact. If I say "Sally broke her leg yesterday" maybe you can guess it probably is still broken but if I say "Sally broke her leg 6 months ago" there's a good chance it's healed by now and past tense gives you no indication of whether it is still broken.
Similarly, to indicate that Sally's leg is still broken, stay away from the participial phrase form (your example #3). Especially in this context, a sentence starting, "have you heard", there is no indication that the action being described is still happening. If I ask you "have you heard about the mariner shooting the albatross?" you should not conclude that the mariner is still shooting the albatross. It could have happened a really long time ago!
The present perfect form (your examples #2 and #5) indicates that something happened in the very recent past so for something that takes as long to heal as a broken leg it should reasonably indicate that the leg is still broken, but to be absolutely clear have you considered the present tense? For instance, "Sally's leg is broken" or "Sally has a broken leg?" Because in that case there would be no doubt that Sally's leg is still broken!
If you say "did you hear" (past tense) it means did you hear at some point in the past? If you say "have you heard" (present perfect) it means did you hear at some point in the recent past? Both mean the same thing in this case.
There is no need for the sentence verb to be the same tense as the verb used in the gerund phrase (that Sally..). I can say, for instance, "I know that you wrote this question." The sentence verb tense should be appropriate for the sentence action and the gerund verb tense should be appropriate for the gerund action.
Note I've included a couple of sources that present perfect implies recent events since one comment on another answer claimed this isn't true. My understanding is there are 3 basic uses for present perfect: experience up to present (often with the word "ever"), recent past, and a recent journey. A broken leg would normally not be experience up to the present when combined with "did you hear" or "have you heard" (who says "did you hear I have broken my leg" if they are referring to their medical history from childhood?) so the choices are recent past or a recent trip. A recent trip doesn't make sense. So that leaves recent past.
"We use the present perfect simple with action verbs to emphasise the
completion of an event in the recent past." Cambridge Dictionaries
Online
http://dictionary.cambridge.org/us/grammar/british-grammar/present-perfect-simple-or-present-perfect-continuous
"The present perfect is often used to express recent events that
affect the present moment." About.com
http://esl.about.com/od/grammarstructures/ig/Tenses-Chart/presperf2.htm
"we often use the present perfect for recent events" wordpress.com
http://englishprojectoxford.wordpress.com/2013/10/24/present-perfect/
Best Answer
The difference is that the first is in the past tense (You hid) and the second is in the present perfect (You have hidden). Neither is incorrect and both mean essentially the same thing.
There are two differences: one is that the past implies a one-time action in the past, whereas the present perfect can refer to a more continuous action in the past. See this tenses chart for a temporal diagram. However, since in this context there is only one possible way to hide a request (you did it one time, not continuously), it means the same thing either way.
The second difference is, as pointed out in the first comment below, that the present perfect "implies that the action still relates to the present". When you say "You hid a request", it sounds like history (not relevant to your immediate situation); but when you say "You have hidden a request", it sounds as if either you just did it, or that you are going to need to make a decision about that past action.